It's controversial and depends on where you live and what variety of tomatoes, as well as how you stake or cage them.. Determinate tomatoes that produce their crop in a short period of time and then are done are not pruned.
Personally I prune out suckers (the little stems that grow out of branch junctions).
Then especially once the plant is getting bigger, I prune out anything that is growing in towards the center. The point is to open the plant up, keep good air circulation through the plant, especially through the center of caged plants. Cages tend to keep the leaves concentrated in small area, cutting down circulation. For those of us in humid climates that have to worry about fungal diseases, air circulation really helps.
People that stake their tomatoes vertically from rope/wire, do a lot more pruning, down to just a few branches.
