I'll give you both sides of the coin; at 17k miles I didn't need to do anything to my shims. I'm now at 30K miles and I'll recheck valve clearances again here soon. No one can really guess how your bike might run if the valve clearances need new shims. If the bike is running fine now then you should be OK for awhile longer. How long? Who knows. Usually you'll see a slow degrade in performance and most of the time its too subtle to even realize. The biggest PITA part of this job was getting the damn valve cover off. It's literally trying to fit a square part through a round frame.