Yes, I've understood that the length of the piper horsehair sporran is also to balance out the rest of the ensemble - the full regalia. To this end, you might want an nicely balanced horsehair like in Matt's photo. I think this is like other aspects of Highland style clothing; go with what you like if you follow the basic rules. I don't think automatic no-no on horsehair unless you're a piper is a basic rule.