i use a cool mist humidifier... if i dont my humidity is in the high 20 - low 30s when i use it it creeps into the 40 - 45% range whitch from what ive read is right about where you want it. none the less im not as expierienced as these other guys so id say "yay" pending on how low your humidity is to start, but like these guys said already too much can be a bad thing as well.

the cool mist wont effect your temp at all maybe even bring it down a degree or two but it also wont raise your % through the roof... mind you im only on my second grow and these guys above me probably know a little better. just wanted to add my two cents.