Personally, if I am in combat, I'll just use all my bullets since I don't have time to look at how many I have left. If I just killed someone and I have about 10 ammo left or below in my thompson, I switch to pistol. Then on next fights I get a few hits in with the pistol and if I run out of ammo with it but I think that the enemy is almost dead, I switch to my SMG to finish him more easily - I usually use akimbos more than single pistol though, so it's much quicker to switch weapons than reload those things.
Like other people have said, you need to try by yourself and see what you can use those last bullets for.