Hello and welcome to the forum.
To answer your question shortly: it's only a matter of style. AI War is a game where, at reasonable difficulty (non-10, in fact), many (many, many) playstyles are possible. You can go archipelago or large empire; you can unlock mostly turrets and go turtle, or mainly fleetships and go big-army, you can focus on starships and play raid (and there is also the golems, spirecraft and fallen sîre, with minor factions), you can deepstrike all the time or get stepstones, etc, etc.
So, if your style is to neuter, eventually gateraid and beachhead the planets between you and your target, so be it. You can also take a remote A-prime planet, destroy the CSG, gather the knowledge very quick and, if there is no controllable things (MkV constructors, etc) abandon it. All you'll forfeit is the metal and energy from it, but provided you would have heavily turreted it (thing you must do on remote planets), it would have costed you more energy in the end. That's two possibilities for the archipelago style. I remember doing a game like that where I captured around 10 planets in the end and, before the first homeworld assault, I was sitting on the AIP floor, maybe around 60 (it was at difficulty 7/7). That's a bit extreme, but feasible, because you'll need only few defenses even on remote, isolated worlds. The less extreme way is to neuter, gateraid and beachhead useless planets "inside" your territory. The problems with this style are resources: you'll have very few metal, knowledge and energy. Metal isn't a terrible problem if you're good in your fights (and with super-low AIP, reinforcements are low, so fights are a little easier); also you probably have a small fleet. Few knowledge means limited firepower: you'll have few advanced ships, but there is no need for advanced turrets and defensive support. I recommend to go either fleet or starships because it would allow capturing only one CSG-B. Remember you can hack for K: neuter and temporarily beachhead an adjacent planet you won't take and suck its K; you have few HaP but the K extractor is rather cheap. Refrain unlocking economical upgrades (harvesters, engineers, advanced OCStations) because you wouldn't have enough metal spots or controlled planets to truly benefit from them. Energy is, IMO, the deepest problem. Having a Zenith Power Generator is invaluable. Another solution is teleporting ships: ships able to intervene everywhere in a blink allow to reduce turret count and tank waves with fleet. MkV turrets are also a huge energy-saver; if there is one on a A-prime planet, you're lucky, but consider hacking one instead of capturing a planet (20 AIP) just for that. (Additional note. I very recently did that with the spirecrafts. It's nearly a cheat: asteroids can be mined without capturing a planet, so you can have a whole spirefleet for 0 AIP and the exo are ridiculous when sitting on the AIP floor.) (Last note. Promise. At damn-low AIP, homeworld assaults are very difficult. You'll probably need to cheese, use several warheads or save-reload multiple times.)
I'll do an "extreme archipelago playstyle" post, one day.
On a more "regular" playstyle, taking around 20 planets sounds reasonable (some players out there have a higher average, but I believe above 30 becomes "rather high" AIP games). When you aim for that kind of moderated end-AIP, taking a specific planet or not may indeed be an actual question. What do you get for 20 AIP? 150,000 energy, some metal, 3,000 K and one spot on the map. If you're comfortable with energy, maybe it's not worth, but if you need some, deduce the energy you'll invest in turrets (if it's a backyard world with few or no adjacent warpgates, you may get a big benefit). Metal is infinite, and even if it has many deposits, remember each deposit is only 20 m/s (un-upgraded); you'd better micro your fights and avoid loses. Most capturables are hackable; it's often a big HaP investment, especially if you play at damn-low AIP level, but it spares you the energy-consuming defenses and the fear of losing it. K is raidable, so if you're sure you'll never need that planet for energy, gateraid, beachhead and K-hack it. Also, a beachheaded world on your frontier is a nice defense, but cost a lot of energy (and provide none). At very low AIP, not beachheading is fine. At medium AIP, beachhead if it's "inside" your territory, and consider only gateraiding if it's on the frontier. If you play archipelago-style, however, gateraid and beachheads are rarely (almost never) useful. A last bit to take into account in the take/neuter question is: all your ships get a x2 speed bonus (more with logistic station). If your fleet need to cross this planet often to defend a frontier on the other side (defend against a reprisal wave, CPA or heavy threatfleet), it might be wise to capture it and put a logistic OCStation on it.
On the other side of the spectrum, there is the "empire" playstyle. There, my advice is to always capture planets in your path. You have bigger needs (energy, knowledge and metal) but better means (large, advanced fleet and turrets). There, every planet "inside" is an economical advantage. The goal is to have the lowest profile: capturing several planets just for the defense layout is fine. You need to use chokepoints and reduce the number of hostile wormhole per frontier planet (one is ideal): that requires taking planets just for that. Gateraid might be ineffective because threatfleet and CPA are a thing. However, with plenty K and energy, defenses are much more potent; don't hesitate to unlock high end turrets and exotic defensive structures (mines, tractor beams, gravitational turrets, fortresses, etc). Save your HaP for sucking remote planets (MkV ships and turrets, eventually MkIV starship/fleetship) without capturing them. Also, abandon lonely A-prime planets: odds are you won't stand the pressure from multiple wormholes (and if/when the OCStation falls, no more supply, no more turrets). Save HaP for downloads too: with a lot of K, you'll need more ship types to invest your K in. (In a word: never K-hack in empire style.)
In the end, is having a contiguous empire worthwile? If it's well done (low profile, chokepoints, Kahuna-style defenses), sure, it's a very potent strategy. However it's not the only strategy and going archipelago is also a viable strategy. And of course, there is the middle ground: a thin and/or scattered empire (or big-island-archipelago) with large profile but few pressure.
Well, I think I covered the most part of the AIP spectrum (I think I might detail more the big-island-archipelago. Human warpgates, transports, etc). I hope it helped; don't hesitate to ask more specific questions. I'll be happy to answer them. Also remember there is more style customization in AIW: superweapon minor-factions, exotic map styles, larger or smaller galaxies (try 10 planets, just once), etc, each requiring its own specific strategies.
