As I was riding along the other day, I worked out that my mare's mane lies on the nearside by her withers, the offside for the next few inches, any which way (sort of bouncy and bouffant) over her crest, and offside up by her ears
Frankly I have given up worrying about where it should be - but when I am plaiting horses I always plait on the offside.
A couple of years ago I went to a yard to show prep a number of horses for the RDS - I spent a long time pulling the mane of a 14.2 pony (while the owner watched) and then I asked her if she wanted it any shorter, at which point she said "I think that's short enough, but it normally lies on the left".
Ooops
He still looked pretty enough to win his class though