hehe, she said mane.
If you're going to a recognized show (Paint, Pinto, Quarter horse, etc.,) You have to cut it. Have to, or you won't even get LOOKED at, lol.
If you're just going to 4-H shows where western pleasure may not be that competitive, you can band a long, neatly kept mane. (Some reining horses have their long manes banded... looks pretty!)