We did a non-ABD tour in Ireland, which rotated the seats daily. It was OK, took the momentary "where do I sit" anxiety out of the equation. But then, that was a tour that showed a lot FROM the bus, so seating was pretty important. My experiences with ABD so far is that the tours happen outside of the bus--nice scenery on the way, of course, but more experiences away from the bus.
With each of our ABD tours, most of the kids end up in back and just love it--and the guides are pretty good at keeping things relatively settled. (Heck, it is a vacation for the kids too, so I'm glad they are enjoying it--I can't tell you how many times I had to nudge our kids awake on the bus in Ireland!). Our daughter and other girls her age did capture seats closer to the front (when she was 7 she wanted to be closer to us--and since there were other girls her age, people kindly let them set close to the front as a group). We rotated around a bit and caught up with others on our tour.
If we had a vote, I'd vote for letting folks find their own seats each day--go with the flow!