From what I have read on here, the general consensus seems to be that you're not going to lose appreciable strength by cutting a thin strip of grain out of the top of a 6/7 oz. piece of leather. Groover vs. creaser seems more important, as a matter of protecting leather integrity, only with thin pieces. That said, some prefer the aesthetics of a crease over a groove, and/or just don't want to remove any material at all. Judging by others' pictures, I think the stitches tend to settle into the leather a little bit differently. I use a groover because that's what I started with and thus what I have, but creases look nice, too. I don't do much in thinner leather so far, working mostly with 6/7 oz. for holsters and knife sheaths and pieces needing similarly thick leather, but I'll probably get a creaser at some point anyway.