A long time ago someone said that cows lay on their left side so the left side brisket would get more work from them getting up, so it would be tougher. I think I have that right. This was attributed to Johnny Trigg, but I'm not 100% sure that is true.
Anyway, I have cooked lefties and righties and can't tell any difference. I find the best brisket I can (thick flat, nice marbling, etc.) and just cook it.