I feel as though people's reactions to this are highly variable, meaning there may be a genetic component. I'm lucky not to grow hair on my back really, but get it on my stomach, chest, and (as the thread is addressing), ass. I've only used razors before (classic gillette-style safety razor). I think the first time I ever did it, the whole area was sensitive after. However, on subsequent attempts, it's been just fine.
Lubrication is key, as always. I prefer using a gel as opposed to a foam or cream (some places call it shaving 'butter' these days). Make sure you lubricate with that big-time, and re-apply often. Even for the spots you haven't shaved yet, the moisture (I suggest doing this in the shower along with hot water) makes the shaving gel run off quickly. It takes a while to do and always feels a bit scary since you essentially do it blind, but after shaving for 1 or 2 minutes it feels quite natural and you get into a natural shaving state that feel comfortable.
I hate reaching back there and feeling like I have two hairy halves of coconuts for buttocks lol. Strangest side-effect I've had is that, after the initial shave (especially the first time), farts escape far easier than they did previously. Isn't necessarily a bad thing, just gotta learn to control it a bit.
The person that said you'll thank yourself the next time you wipe your ass is 100% correct. Smooth sailing. Gotta hate feeling like you're trying to pick pieces of shit out of your ass hair.
Afterwards, I always apply a basic moisturizer to the area. Gives you a slight feeling of swamp-ass for that first day, but don't worry...it's just the cream, not actual sweat lol.
Happy cuttin'!