I've cut a pair or two of bars before with no problem. I think the only important thing to it is using some sort of hose clamp, zip tie, etc to make sure you're making a "straight" cut.
I'm wondering though, if you were going to cut some chop-n-flop bullhorns out of a drop bar how would you go about making sure one side was being cut at exactly the same place as the other? I've kind of gotten it to work using a measuring tape, but it's not exact and I'm not satisfied with the job.