I don't think I've seen the AI do that, but better to be safe than sorry. Don't completely neglect defenses on colonies the AI can't reach yet, because eventually they will get the fuel cells they need to reach it. And when they do, they will strike you where you are weakest.
If you still feel that you don't want to build defenses on those planets, at the very least make sure that the planetary shield is built so that you don't find yourself unable to build any missile bases when the computer catches you off guard.