Your valve clearances are your indication of wear at the valve face. Usually, the big YZF is set up when the valves are installed, and they then don't need an adjustment for a very long time. Check them periodically. Once they start closing up, start keeping closer track of them. For example, if a valve that started out with a 180 shim one day needs a 165, it's a good indication that it's time to replace them. Stainless valves can be used, but I don't see them being any more reliable or durable than Yamaha's OEM Ti valves are.