I work for a charter airline and very very rarely get any pax say to me "Keep the change" and even if they do, we can't. At the beginning of the flight we have to confirm what personal money we have on us, so that if we get audited at the end of a flight they can see we haven't stolen any money. Therefore if we were to accept tips, this would be seen as stealing.
People in restuarants don't think twice about tipping waitresses, and although we are not waitresses, we do sooo much more for such little pay in some cases. I don't think it would be wrong for more pax to tip us!!