Hi Tia! How many hours of sunlight are the plants getting?

"Is it necessary/advisable to pinch back to prompt branching? TIA"

Not at all. In tomato horticulture it is called pruning rather than pinching back. And doing it isn't required for any tomato type/variety even indeterminates. It is merely an option that some choose to do because of limited room.

However determinate (aka bush) varieties are normally not pruned because you lose most of the crop by doing so. There is a FAQ here about it.

Like Chris said something else is causing your plant to grow as a single stem rather than to bush. Possible causes are over-crowding, insufficient sun exposure, leggy as a seedling due to insufficient light, etc.

If you could post a photo it would help.

to-prune-or-not-to-prune FAQ

Dave