The slightly irregular un-stitched edge is called "selvedge" and is a locked edge created by the weaving loom, where the edge threads are folded back into the body of the fabric to prevent it fraying. Without this edge you'd need a hem or the kilt would eventually unravel. It is possible to hem a kilt but I wouldn't cut away the selvedge to do so unless it's unavoidable. Paul Henry (kiltmaker) could maybe give you better advice on how to handle yours (I'm no expert). Maybe he'll be along presently or you could PM him.

Best Regards
JOHN

...of the Clan MacLately.