Depends. If I have nothing to stow under the bus, then a tip isn't going to be expected. But, if the driver 'touches' your bag(s), then he/she will be looking for a tip.
Now..with that being said, here are a few of my DME experiences.
1. had a bag that was stowed beneath the bus..carryon size but too big to keep with me on the bus. I had $3 in my pocket for a tip when I got to my resort. BUT..that driver never said hello, welcome to Orlando..nothing. He said nothing when he got on the bus. The only things that came out of his mouth was to announce which resort we were at, and...wait for it...ready??? To tell us that gratuities are always gratefully accepted!! I kid you not. There is a sign, right above the windshield, that states this policy. Nope, that $3 stayed right in my pocket. And the driver had a not so nice expression on his face when he handed over my bag. But I'm sorry, he did nothing to make my bus ride nicer....in fact, he made it worse. He held everyone on the bus until all bags for that resort were taken out.
2. had no carryon bag to stow, only a tote bag that I kept with me. But, this driver was hysterical. He told us little bits of area history, he was funny. He was a safe driver. He was quiet when he should have been...no talking during the video. He was so pleasant and cheerful. He seemed to really enjoy his job. He got a nice $5 tip when I got off the bus. I told him that he had started my WDW stay off incredibly nicely!!
So, you can see that different experiences will have different results. But, long story short..if they touch your bag, they'll look to be tipped. Otherwise, not so much.