You at least need something to stop your axles from banging on the frame. Those ong bumpstops are really stupid, and limit your travel a bunch. Only use as much as you need to stop your tires from eating body parts. A good way to do this is to drive a tire up a ramp until your tire is about to make contact, then measure from the axle to frame, and do a stop that is that long.
everything Brad said...
plus this - flexing leaf springs (the lift type with positive arch) past the point of flat is flexing them too far and will lead to sagging then over time/repetetion - failure. May or may not be an issue to you given the use for your Jeep... but worth keeping in mind when setting your Jeep's bumpstop length.
and this - make sure you have enough shock travel in the compression direction. i.e. you don't want the shock to act as a bump stop.
also for coil springs (I know you have a YJ but maybe someone could use this tid bit) you need set bump stop length so that the coils do not compress so far as to make contact. Important point.
sometimes having a tire meet fender is only part of the equation. with no bumpstops, you could compress the front or rear end to the point, where the driveshaft has no place to go, but thru the transfer case. i would run bumpstops of some kind.