On the few mass start charity-rides I have done (never a race) I expect it to be just short of chaos at the start then thin out fairly quickly. Since I have no need/delusions of finishing in the first few I always position myself near the back at the start. I go EXTREMELY easy for the first mile or so as the thundering herd begins to thin itself. It's true that putting myself at the back means there are more folks in front of me to weave through as I pick up the pace, it just seems to make life less stressful for the first few minutes.