No matter what type of classes or shows you are exhibiting in....all other things being equal....the most polished and professionally presented horse will be more likely to get the nod. (the only exception to that might be classes where performance only is what counts like jumping or barrel racing, However, even classes like that have certain grooming expectations)
I have shown, & won, in open halter classes with a horse in unclipped summr coat, but these few horses had very fine haircoats and were otherwise trimmed & groomed to the max.
And if you clip you want to always leave a tail 'V'.
Charlotte