Some people will say you don't have to, but everyone that I have ever dropped with a flip kit in the back will eventually bottom out on the frame. If you can live with that, you don't have to. Putting anything relatively heavy in the back is going to make it sit on the frame though.
I ran a 4/6 drop for years with 275/60/15's. you may rub on bumps on the fronts with a 5/7.
BTW, any current drop company sells quality pieces. Belltech included.