Tamworth Yamaha did that to mine at the 6k service, it was practically humming under tension on the way home, where I wound a couple of full turns out of the adjusters. I went elsewhere for the 12k and 18k services. They were useless in various other ways. Then the warranty ran out. It hasn't been back to a dealer since.
I did have a fair amount of work done under warranty, and while they *should* do warranty work regardless of the service history, it removes one way they can try and wriggle out of it. So it was probably worth it for that, but as far as the servicing itself goes, if you're even half way competent with the spanners you can probably do a better job yourself.
It's worth pointing out that 'resale value' doesn't enter into my thinking. I've abused my bike enough that if I was going to sell it, the lack of service history would be the least of my problems
