I only tipped the bus driver one time -- that was the first year I went (2007). It was when we were nearby eating dinner on Saturday night and I was ready to go home (or back to the hotel!) so the bus driver took a bunch of us (not a scheduled run - an earlier one!) back to the hotel (I was tired). So I tipped him because I got home earlier than everyone else. Tipping is not mandatory in this situation. I would just play it by ear and see what happens! The bus drivers are always super nice, so most of the time you would like to give them an extra in tips! My opinion!
Julie