I have used DME about 5 times now. You are only expected to tip when, and if, the driver helps you with your bags. No bags, no tip is expected. But....there have been times when I have had no bags and I still tipped the driver a few dollars. He was a great driver and kept us entertained the whole way to WDW. He never mentioned tips. However, I have had a driver who, although he handled my bags, felt the need to mention the fact that gratuities are gratefully accepted, in addition to the signage right beside him on the dash board.
It's up to you, the guest, to decide if the driver did something to make your experience nicer. A tip of about $1 per bag is normal.