I never did tummy time. With my first, I tried it and he just cried, and I do not believe that crying teaches babies anything, so I stopped. With my second, I didn't even bother. He was worn a lot, but I didn't stress out about it at all. Both my boys have plenty of upper body strength (too much for my tastes ) and were both early walkers, so I think I did something right.
I have to wonder what people did before we had helpful doctors telling us that we need to put babies on their tummies for X minutes a day. Somehow I doubt that they all ended up with delayed upper body development.