I use DME several times a year. Here is how I handle the DME bus driver tipping issue.
If the driver has contributed, in some way, to making my trip to WDW more pleasant, then I give a tip....whether or not I have had a bag stowed. If that driver has done nothing more than open the bus door, drive the bus, and then let me out at my resort? Nope, no tip. Especially if that driver makes an announcement at each resort that gratuities are always gratefully accepted. And I don't much care if I had 4 bags stowed...you ask for a tip, you get nothing!
I have had drivers who were wonderfully informative, and were funny...all while safely driving the bus. They made my trip with them very pleasant...they got nice tips (about $5 for just me). But, I have also had drivers who never greeted me as I got on the bus, they never said a word on the drive to WDW. Then, I actually had one driver who was almost surly....no greeting, no chatting on the drive...who stood up at each resort, and announced that gratuities are gratefully accepted. He then got off the bus, closed the door behind him and wouldn't let anyone off the bus until all the luggage was unloaded at each stop. He actually had his hand out...literally!!!
I really take each trip on it's own. I always have tip money in my pocket, and usually I hand it out at the end of my ride. But there have been several occasions when that money stayed right in my pocket.
I don't much care how much the drivers are paid. That isn't my problem. Waitstaff get paid only a few dollars...they depend on tips. Bus drivers??? Nope. If they want a tip from me, they are going to have to go beyond what I consider to be routine service. And, there is signage, at the front of each bus, that you can't help seeing as you get off, that says...'gratuities are gratefully accepted for outstanding service'..or something like that. I do NOT need to be told about tipping.