I would do all the way around (excluding the bottom). T-molding is cheap (if you aren't me). And it's easy to install (especially on the back). While people will not generally see the back of the cab, they will never see the bottom of the cab. Those are two very different things that, IMO, separates t-molding on the back from t-molding on the bottom. When you do see the back of your project it will look unfinished. I say why have something that looks unfinished when you could finish it for a couple of dollars and fifteen minutes more work?