I have roached my mare's mane a lot, to keep it out of the way while barrel racing. Plus, when a horse has a nice neck, I like to show it off!
I have roached my two year old's mane one, the vet had to shave off his forelock to stitch up a cut so I just did the rest of it. He looked pretty good, too.
I believe the corret way to roach a mane is to leave the forelock and about 2-3 inches at withers. To me, it just looks stupid that way, though.