I think it depends on the horse. My Dun QH has a short neck and his mane just looks silly long so I cut mine. I show him at open shows and stuff so I guess its more proper.
I'm huge into reining tho as well and there is nothing prettier then a horse with a goregous long well groomed mane. I think palominos look best with long manes. There is so fine and straight.
I have probably a stupid question about pulling. I always pull my horses mane but I don't think I'm doing it right. I take the longest hair, few at a time and yank them out in a downward tug and the hairs come out at the roots. So my question is i get a thinner mane but not shorter. then I have to cut it to the length i desire. Is that what happens or I am doing something wrong?