I've done a couple of mass start charity rides,and the first one I started near the back of the pack. It was a bad decision. There were many inexperienced riders on this charity rides, and the first 5 to 10 miles was disasterous. The route was very narrow with many intersections, and I watched many crashes and near crashes as people couln't hold a straight line and ran into each other. Since then, I always start as close to the front of the group as possible. I try to go out fairly fast for the first 5-10 miles, then settle in to a comfortable pace. I figure if I am an average rider, half the field will pass me before the ride is completed, but in the mean time, i don't have to deal with the other half of the field.