For what it is worth--
Mine was in for ignition recall last week and it has 24,000 on it. I mentioned the 26k valve check coming up, and the head mechanic said the same about not needing it, as he has never had one out of spec, but that he would take my money if I wanted to spend it. He said he'd leave it alone if it was his bike unless it wasn't running fine.
I mentioned my YES is good for another couple of years, and would it affect the warranty if I didn't have it checked at 26K? He said they (Yamaha) have no way of keeping check of service intervals. I could have one dealer check it at 26K (or not) and bring it in to him at 50K with a problem, and they would not know if it had been checked or not. So if it was beyond normal wear, as stated in the warranty, they'd fix it under warranty. If it was normal wear, they would not, per the warranty.
B)