When do you consider dome repair?

For those of you diving with acrylic domes, when do you consider repairing it? I see there's an excellent thread on this forum about products and process of the repair. But when do you decide it needs repairing? The process seems scary (at first!), so wouldn't want to do it unless absolutely necessary. I do see some microscratches on my Seafrogs dome but they don't really seem to show up in pictures. Is there any signs to say that yes - it's time for a repair?

Small scratches that don't show up in photos - don't bother, water smoothes them out while you're submerged. Larger scratches - yes, they may warrant polishing, or you could just get a replacement sphere from SeaFrogs for $80, which is not that much more than a micro-mesh kit.