If a driver handles your bag, a tip will be expected. But.....here's my experience.
I had a bus trip, from MCO to BWV, with one stowed piece of luggage, a carryon bag. The driver took my bag, never said a word. He got on the bus, after stowing all the bags, never said a word. All the way to POR, never said a word, other than to announce that we were arriving at POR. Then, he stood up, saying that gratuities were always welcome. Same thing at YC, BC, and then finally BW. That driver never said a word, other than his gratuities spiel. There's signage, right over his head as he stood there, stating the exact same thing. As I got off the bus, not a word was said, nothing. He got my bag for me, and I walked away. Now, I had had a few dollars in my hand to give him, but after after he asked for a tip? Nope, not happening.
However, there was another bus ride, completely different. I had no bag to be stowed, just a tote bag that I kept with me. The driver welcomed everyone to the bus. When he boarded the bus, he again welcomed everyone. Then, he chatted about things in the area, all while driving very safely! He was a hoot, giving a great start to my trip. The onboard movie wasn't working, so his chatting was great. As each person hopped off the bus, he thanked them for riding with him. Well.....as we approached my resort, I started digging in my bag for a tip. That driver got $5 from me that day...with no bag touched. He was thrilled and very appreciative.
I will not tip when asked for one. Tipping is never mandatory!! Ever. Yes, there are times when we are expected to tip, but a tip should never be asked for!!!