As was said before, there's a lot of opinion on this - so, the best I can do is pass along my experience. I'm running about 4" of lift and have 32x11.50 MTR's on 15x8 wheels w/ 4.5" backspacing (and a 4.0 w/4.10 gears). "Normal" driving was fine, but once I started to do some serious rockcrawling - I saw that I couldn't get away with it unless there was some trimming up front.
At full stuff and turning, my MTR's were grabbing the lower back side of the front wheel well fenders as well as pressing up against the sharp inner seam (not good for the treads). The tires also had the unfortunate effect of grabbing one of my brakelines and twisting it up a bit.
Can't say what the effect would have been on the lower front side of the wheel wells - since I had already trimmed that part away (everything below the turn signals) to fit on a custom front bumper - but it looks like that would have been an issue as well. Did some "field trimming" of the wheel well in the campground at Moab, shifted my flares up about a half inch or so, and repositioned the brakeline (should have done that earlier anyway).
The rear wheel whells (I have a 2 door) handled the tire/wheel/lift combination fine.
I don't think it requires massive trimming, but trimming is definitely required, and will depend strongly on what backspacing you use. At stock bs, there's probably less impact on the outside fender, but it will start to affect the control arms and inner walls more. Less bs will affect the fender and flare more. (for me 4.5" was a good compromise).
And for me, 4.10s w/32's is working OK. Good street performance and acceptable crawling performance (with a 5 speed).
Good luck.
Mike in NJ

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