We clean bags after every trip. Inside and out. The more I travel the more I dislike that strange, stale hotel smell! Why put clean clothes into a dirty bag? We also make sure that we haven't picked up anything that might soil the clothes we are wearing when we haul the bags. One bag came out soaking wet after a 3 hour trip in an overhead bin. Something in someone's bag had leaked.
I really scrub down hardsided luggage.