It depends on what rules system.
Well, in amateur boxing use of shoulders is prohibited, but in pro-boxing circles, limited use is allowed.
For example in the clinch it's allowed to use your shoulder to push your opponent, but it's illegal to use your shoulder to strike with. It's allowed to use your biceps/shoulder to push away your opponent in in-fighting but if you hurt your opponent using them, you'll be penalized for it.
Limited use is an option, yes.
I got two fists.. Don't make me use my head as well!