For years Cabin Crew have justified their position based on their SAFETY role.
If you start accepting tips, you will diminish your standing in the aviation community. Wages WILL go down commensurate, as you will be seen completely as a waiter/waitress.
Maybe I am old school, but it is completely unacceptable to accept a tip. Allow the passenger to make a donation to the onboard charity (if you have one) in its place.