It just so happens I gave my opinion on the subject in another thread, I'll just copy/paste here.
Put it this way, I have a PML and a 2 inch BL. I kept the stock rims with 4.5" BS and put 33X12.50 R15 tires on them. I didn't trim any metal but the plastic had to be completly cut out of the way of the metal under it to keep from catching and possibly ripping the plastic off. In my case, any less back spacing and I would have to cut metal. With the front wheel fully stuffed (as far as I could get it) and the wheel turned (as close to the rear of the wheel well as possible) I only have about 1/8" between tire and metal. I'm sure If there were stress on the control arm bushings it would make contact, and that is as far as I wanted to push it without trimming metal. Also, I still have to adjust my bump stops because the tires make contact with the frame behind the control arms. Hope this helps you out.