I can't afford the suspension I want right now but the stock Goodyear Kevlars are a joke and I had to upgrade before I spent any real time on the trail, so I opted to upgrade now anyways knowing that in short time I'd have to change them again, adding cost. Some see it as unnecessary but it all depends on your timeline and how you use your truck.
Generally speaking, 265/75R16 or 265/70R17 are the largest tires you can fit stock but you may still have some rubbing depending on your alignment specs, the backspacing/offset of your wheels, and a variety of other factors.
If you plan to wheel the truck until you're able to upgrade the suspension, I think it's worth every bit of whatever you may lose on the resale of the tires to upgrade now.