If he helps you with any bags, it would be customary to tip $1-2 perbag, $5 for any bag over 50 pounds.
If he does nothing for you except drive, then IMHO tipping is not necessary. Some of teh DME drivers have been known to tell passengers stories, point out attractions, whatever, and in that case if you feel he's gone above just driving, it would be a nice gesture to tip $1-2 per family--not person. You shouldn't feel obligated to tip unless they helpw ith bags though. And if they are agressive about getting a tip, I wouldn't tip at all.
Anne