From what I'm reading from responses, it appears that neutering is not absolutely necessary and whether to neuter a planet should be decided on a case-by-case basis.
I have a lot of concerns about these bordering planets and, since this is my first game, I'll probably exercise caution and neuter them, mainly because:
1. A lot of the planets that I'm neutering are in the rear of my empire, and the AI's buildup along my borders leaves me vulnerable to a sucker punch from the AI when my guard is down. I'll probably continue the practice of neutering all adjacent planets because as I move forward, those planets along my front will eventually become my rear. I guess that I fall into the cautious category of players, eh? I don't plan to neuter any planets that are more than 1 hop from my own since these planets would not be on alert.
2. I would not likely be able to withstand an assault of over 1,000+ ships from these bordering planets. The planets that I'm targeting to neuter are all MkIV while only a small portion of my own current fleet is Mk3. I believe that I have sufficient turrets and mines to hold off these attacks for a bit of time, but I'm beginning to see such a large number of reinforcements that, without neutering, I'd either have to build up my rear guard fleets to over 500 to 700 (or more) on the main planets, or I'd need to take a huge risk in moving my fleet forward. Are you folks OK with the AI building up over 1,000+ ships on its planets that border your own, even though your main fleet may be 6 or 8 hops away?
3. I'm trying to move the majority of my forces forward for exploration and adventurism and, as mentioned above, this would leave me susceptible to rear attacks. It takes time to neuter these planets, yeah, but I can do it without losing a lot of forces if I'm patient. I have a hunch that if I take the time now to stabilize my borders--and don't wait for the AI to amass 2,000+ ships on it's planets behind my front--then I won't be forced to withdraw my forces and defend my rear at the exact moment that I'm attempting to stomp the AI homeworlds. It'll slow my game down for a bit while I clean up the mess on my back porch, but hopefully this will lay the groundwork for a strong offensive down the road.
It'll be interesting to see how quickly the AI reinforces the neutered planets. Without being neutered, I saw a number of planets jump from 400-500 ships to 1,000+ ships within what seemed like a half hour to an hour of game time.
H.