There really is no magic answer. It really depends on what your using for your lift.
Some of the lift kits use a drop bracket that is right above the pumpkin and is what puts the hole in the diff.
I'm using drop brackets from a Duff 2.5/3.5" lift kit and I still use the factory bump stops. I have even used the f-250 upper shock mounts so I can run very long shocks now and even with that I don't hit my pumpkin on anything. In my case the tire hits the airbox/bottom of the fender right about the same time I run out of spring compression and shock travel.
~Mark