There are a few fighters that don't cut weight. I think Maynard puts on weight to make 155, lol.
Most fighters cut at least 10-20 pounds though to drop down a class -- ie walk at 180-190 but cut to 170 to make WW. Most fighters don't walk at exactly the limit of a class, and cutting a few pounds to drop a class isn't too difficult/taxing for most athletes, and can give an advantage against a smaller opponent.
It can be taxing on your body, but if it's done responsibly it's not too bad. If you attempt what Leben did against Munoz though, then that's another story. 20+ pounds in under 24 hours before weigh-in's is just asking to be gassed a day later at the fight.