I have this young Fragrant Sumac shrub. Do I let it go or do I prune it now to encourage branching? https://i.ibb.co/d5KDwqr/IMG-20190608-195336317.jpg
In part that depends on what shape you want it: Tall shrub, small tree, or short shrub.
Sumac can be a tree. By normal habit it's a 6-12 foot high shrub. Ignore it, and that is what you will get. Tends to be irregular in form.
If you want it to be bushy, take off the tips. Side branches will grow instead.
If you want a more tree form, remove 2 of the 3 stems, then later remove side branches from the bottom third of the tree until you get to a height that allows you to mow underneath. You are unlikely to get a real tree out of it, but you may be able to shape it into a dwarf tree that could be quite handsome.