I have personally seen HITs that require certain low levels of abandoned HITs. I have never seen one that had anything to do with returned HITs... I will return a HIT in a heartbeat - but sometimes I do get abandoned ones. Usually it's a crappy situation where I refreshed a page and accidentally accepted an extra HIT I didn't realize or I get so caught up in one set of HITs I forget about another... I would say that if you can, RETURN a HIT before you let it Abandon. But in general neither really matter too awful much. Just your Approval Rate!!! Keep that above 99% if you can! Anything below 95% can be catastrophic and too much lower than that (I don't know the number) Amazon mTurk will be in contact with you about an account suspension!