I would really say that depends on the situation in the rest of the solar system. Apologies for being vague there, but basically if they are fighting one another a lot and/or having low armada counts for some other reason, then you can push outward more quickly. In particular, you could take down some potentially deadly foes early, like the Burlusts or Thoraxians, if there's an opening.
If everyone is strong and growing stronger, and not doing much focusing on one another... well, if they all turn on you you're going to die. So a big part of this decision is figuring out how you're going to manage their attention. If you are in a situation where they are suitably scary, personally I hang back until things feel more settled.
Snapping up the very weakest planets doesn't tend to be too useful (I don't think) because that doesn't really give you a good return on income compared to the ire you generate... though some players probably super disagree with that. I'm just a cautious sort of player, really.
Anyway, my own thought is to try to get discord between the other races that outweighs what they feel for you. Then snap up the most theoretically threatening races, and/or races with medium power, and/or races that aren't a part of the discord and thus which would grow strong in between the carnage same as you will.
The TLDR, I suppose, is that it's not just about managing your own expansion. It's about making sure that everyone else has so much fighting with not-you that your actions are lost in the background noise and you aren't the one being hammered on constantly.
