Guards can be great on a monk, but they are obviously better if you're getting hit more. In non epic quests, I would make them more of a minor bonus and not build specifically for them, but if I can fit them in, all the better. In epic, you'll be getting hit nearly as much as any other melee toon so they will be of more use there.* It all comes down to what slot you have it in and if there is anything better for your situation for that particular slot.
*Depending on how you play your monk in epic, you might not get hit all that often even if AC is a non issue. Shadow fade for starters will put you at a 50% miss chance. Skillfully played monks avoid initial aggro and instead pick off targets with stuns, light monk instakill/stun, quivering palm or just a burst of damage from a well timed ToD. Even the best played monks will be getting hit in epics, but the point is that a barbarian will still likely be getting hit more, given equal levels of gearing and playskill.