1/8" isn't a lot to get worked up over in a growing horse, IMO. However, keep an eye on it w/good dentistry and make your decision based on what she ends up at.
I DO think bites are genetic, though there are some that may just crop up for whatever reason, out of parents that are both just fine.
Have a look in her parents' mouth if you can, and if there's any sign of a problem, it may be that there is something genetic in there and you may want to choose a different occupation for her than broodmare.
If it stays 1/8", though, or gets better, I wouldnt' worry a lot about it.
Good bites are important to me, and all of the horses I have bred except one had naturally good bites. The one mare I got her bite went back on when she was five, on its own, and she had a filly that had a pretty severe underbite. The filly needed a lot of dentistry and the dentist I use felt it was congenital even though I did not tell him about the mare, etc., said it was the way her skull was built that crowded her jaw (her wolf teeth came in directly behind her incisors, for example).
Good luck and I hope it works out great!
Liz M.