First of all, I assume that you are refering to the brush guards over the periscopes.
If so, it depends on which variant of Sherman you are modelling as to whether or not they become optional. If you are building the M4A3, yes you would need them. But if you are building an M4, then not necessarily.
If you're building the M4A3, follow Jon's advice and incorporate a little battle damage to make up for the broken piece.
Happy Modelling and God Bless
Robert