Alot of knuckle lifts will rub on the rims but not all do. Rough countrys site will say on its page for the lift the rim or spacer requirements needed my 3.5 rough country knuckle lift needed a .25 inch spacer for the wheels I put on 1.5 in the front and 2 inch in rear to even out the track width. Spacers are fine as long as they are hubcentric and torqued properly. Just remember the bigger the spacer or offset of rims used the wider your scrub radius will be during turning and the worse rubbing will be. My truck didn't rub at all with 33s before lift after lift they scrubbed behind the bumper because of the spacers I just trimmed off part of the liner but it can be heated and reshaped to clear easily.