it sounds a lot cleaner and nicer if they're the same note. if that's not quite possible (for instance because your key and bass notes are so high or low that the kick sounds weird) a fifth is fine too. i wouldn't use any other relationship between the two (except when the bassline is moving around following the chord progression, then it sometimes sounds better when the kick doesn't move as much or stays put).
btw: it's also fine to have them play the same note if they overlap. just cut the low end of the bass when they're playing at the same time (in a psy context this is most likely just the first bass note of a kbbb)