Well the metal expanding inserts work better than the rubber ones. But even when I tightened the heck out of them when I'd adjust the mirror it would wiggle at the mounting point. By welding the nuts I have really solid mirrors, no movement other than at the actual mirror pivot point for adjustment. Wind doesn't move them anymore either. I don't ever plan to remove the bar end mirrors as I love the look and IMO you get better visibility, but if I did the nuts could have a black bolt threaded in to hide the nuts. Or just leave them open.
Whatever you're happy with, just sharing my experience with the different mounting options. I'm picky though.