Combat drops rep even more. Best to raid without a fight if you want to preserve rep. For some worlds, it is too easy (especially New Maxios). Others have constant patrols if there are no pirates, and I bring a pure phase fleet to sneak in. If I need to raid a world multiple times to get all of the blueprints, excessive combat is the last thing I want.
If player needs to raid many times, then he will need to wait after few raids anyway to let stability recover, if he does not want to decivilize the planet.