The 33x12.5r15 ProComp Xterrains on my TJ stay perfectly balanced until I go rock crawling with them aired down at 12psi. When you have them aired down and rock crawling, you have a small chance of being able to spin the tire a little bit on the wheel which ends up putting it slightly out of balance. After half a dozen times off-road rock crawling I noticed my tires were starting to vibrate a little at 55-70mph, so I went to re-balance them. They ended up being about .25-.50 oz out which isn't that bad at all.
If you never air your tires down really low or do any hard core rock crawling your tires should never come out of balance (assuming they were properly balanced the first time, its very difficult for 95% of the people that work at tire shops to balance 33"+ tires properly. You usually only will have good luck getting it done the right way by having somewhere like 4WheelParts balance them).
Make sure you also rotate/cross rotate your tires, keep the air pressure constantly at the right psi and keep your alignment in order (along with keeping your suspension up to par, bad shocks will kill your tires).
2000 TJ - 2.5l 5-speed 4.10: Airaid Intake, PowerAid TB Spacer, Banks header, Magnaflow Street Series CB & Performance Cat, Jet 91 Tune, Rancho RS9000XL, Dynatrac Pro Series covers, Smittybilt SRC front & rear, ProComp Xterrain 33x12.5r15 on Eagle Alloys, Power Slot rotors & Hawk pads, Rhino lined interior, etc...