Hi There !!
I used to have a mare that was freeze marked on her shoulder and she always did extremely well in her showing classes, and when she did badly it was because of her awful behavior and nothing to do with her freeze marking , or where it had been done.
I chose to have it done on her shoulder as she was so thin- skinned ,Im sure it would have been ages before I could have saddled her up again. !!!! she was also a fiery chesnut mare and Im sure she would have dragged out this excuse for not being ridden till hell froze over !!.
I also judge showing classes both ridden and in hand and I have never marked an animal down for having a responsible owner.
I also show my native and show ponies at county / breed show level and I have never seen a pony or horse marked down either.
When a non grey is freeze marked the hair will grow back white as the colour pigment in the hair follicle is destroyed, with greys or when the mark is branded on a coloured horse where the hair is white the brand is left on a bit longer and it kills the hair follicle completely and no hair grows back so its always wise to keep this area clipped , esp in the winter when long winter coats cover the naked skin.
I hope this has helped a bit
Love Ally