I have used DME more times than I can actually remember. I have tipped when I've had bags stowed, I've tipped when I had no bags, but on the other hand, I have walked away, tip money still in my pocket, when I have had a bag stowed. Why??? There is a sign, right up front, above the windshield (pretty hard to miss!!), that states that gratuities are gratefully accepted for exceptional service. And that's fine. But, don't get up and announce, to a bus full of people, that gratuities are always accepted. Especially after there were no welcome greetings upon arrival at the bus, no 'have a nice day' or 'thanks for riding with DME' upon departure from the bus. Nothing...nothing other than 'gratuities are always accepted'. Yeah, you get nothing my friend!!!
On the other hand, I've had a hugely welcoming bus driver...he was so friendly and happy. He gave us tons of tidbits about the area as he very safely drove to WDW. He never asked for a tip...didn't mention it one time. He very clearly announced where we were headed to next, enabling the guests on his bus to fully understand whether or not it was their stop. Other drivers tend to mumble or don't speak clearly..so you can't be sure unless you are familier with the area. That bus driver got a $5 tip from me. He was incredibly grateful..and thrilled.
So, it's up to you. If a driver 'touches' a bag or other item for you, they will be expecting to be tipped. It's up to you to decide if their demeanor warrants tipping.