With the 0.76b.018 version, the culture setting is fine with me. I don't have a problem if you conquer a planet/asteroid and after colonizing it, it reverts back to its old owner. I do have a problem where it instantly reverts back to the old owner. Make it more realistic...like it yours, but the allegiance slowly falls to 20% and an uprising occurs and the planet reverts back to the more powerful culture, unless during that time you manage to destroy broadcasting stations in adjacent systems or build more broadcasting stations of your own.
Thus this give players a way to win planets without fleet battles.