I roached my OTTB's mane last week. It's a little ratchet because she was still getting used to the clippers. I roach her because her mane is so thick and black. Prior to roaching the mane, she would sweat like a madwoman under her mane. I always make sure she is fly sprayed and comfortable. I see nothing wrong with it.
I roach my TB every spring. I hate manes. I keep his trimmed in a gentle arch just a couple inches long in the middle. I do let it grow it all winter but come spring, it's gone! Posted via Mobile Device
Here's his mane, first pic is a few minutes after he arrived. You can see that he's got no bridle path and it flips to the other side before his withers. The second pic is after a measly pulling job. :P Sorry his heads not level. He's super ADHD right now cause he's in a new place, and I didn't feel like fighting him. Also he has a beautiful tail (last picture).
On a horse who's neck is not-so-great, instead of a straight flat roach where you take all the hair off all the way down to the mane base, you can trim it in an arched fashion, trimming it short at the poll and toward the withers, but leave it a bit longer in the middle. That can give a horse with a bad neck the appearance of a better one.
Think of how a fjord horse's mane is usually trimmed.