Well, if you look in the wiki and search the forums about "neutering" planets, you'll find a lot of discussion about just this sort of thing. Opinions vary.
I think that it can be a good thing to neuter some of the AI planets (kill all the guard posts but not much else), but if done to excess then it winds up working against you. The number of reinforcements that the AI gets are primarily a factor of time (as well as how many planets they do not control). So if you're burning lots of game hours on killing reinforcements on some planets, then that just means that the AI has more time to build up elsewhere.
Well -- that's referring to actual reinforcements, which don't count as threat/attack value. All those guards at guard posts, is what I mean. Reading your post a bit more carefully, I see you're actually referring to threat ships, which see you under attack and convert themselves into attackers when you're most vulnerable. THOSE guys tend to be clustered around the other side of your wormholes, not in low power mode even while waiting, and killing them is a whole other matter. My opinion: go for it.
If you've got a threat count that is spiraling sky-high, especially on particular planets (there's a galaxy map filter for that, if you have scouts in place), then you want to defuse that. That's really different from neutering, and it's not something that you'd need to engage the entire planet in order to deal with. You just want to kill those threat ships that are hanging out by your wormhole waiting to skewer you. Pop in, kill them, watch the threat number go down, and get out.
When it comes time for a CPA, the same number of ships are going to get turned into threat no matter what you do -- so neutering planets won't defuse that at all. But if you've already
got a high threat count, and then the CPA adds tons more on top of what's already there... that's when you're getting steamrolled. And it's those high concentrations of threatening ships before the big waves or CPAs that you can more easily defuse. If it's just a few ships scattered here and there, it doesn't matter -- if there's hundreds of ships stalking you from one planet, that's just a ticking time bomb. It's not like you want to go over there and sweep every hour, but it is a good idea to make sure you're not routinely in mid triple digits on multiple hostile planets.
In terms of what you're doing now, there are definitely downsides to that, but it's also very possible to win that way -- but you have to have a rock-solid defense. At some point, yeah, the AI is going to come smack you really hard, and you've got to have a bottleneck ready where you can crush them before they get you (or whatever other strategy you prefer, but I'm a bottleneck sort of guy). The main thing not in favor of your current strategy is the time and resources your spending on neutering entire planets at a go. It can work really
well if you do it right, but it's a longer form of game and it's tricky because that's going to make the AI reinforce that much more away from your border worlds, which can cause you problems later on when you find those non-border planets more built up than you expected.
Bottom line: play how you like, and what you're doing is definitely valid. But my personal preference is to do those sort of aggressive raids only on some planets, while raiding the threat ships specifically on the majority of border worlds, and then focusing more on actual expansion and offense for the remainder of my time.
Hope that helps!