IMHO, irregular jimping will add to its looks. Unless they are all over the place and look like I did them! Part of the beauty of these custom things is that they are hand crafted and unique. I love that my strop is hand made and stitched (not by me I hasten to add) and that it will be different from every other strop that person made, rather than identical to the 1000 others machine produced that morning.
So slightly irregular jimping will not put me off, however, for someone like yourself who is maybe a perfectionist, or close to it, then I hope you find a checkering file (I am off to google that one!) soon.