You've gotten some good answers here - suffice it to say, that neither of my DC ever had "formal" tummy time and they do not appear to be stunted.
In fact, I think most people I know never did tummy time and their children are, by all accounts, developing normally.
I wish that the peds would just adjust their developmental timetables to account for the fact that when they were designed babies were routinely put on their tummies to sleep and so learned to roll earlier (kind of like the formula-charts, KWIM?). And has it occurred to anyone that the reason babies learn to roll off their tummies is because they don't like it?? (I actually think it's some other reason, but this seems plausible
I think it is true that babies often do spend too much time on their backs, but that's because they are too often not held enough. Doesn't sound like a problem for you