These tools (like YSlow) are oh so interesting when your clients install them in their browser and examine the site you built for them. Now, I'll get to hear complaints that we used too many inefficient CSS selectors (and that we're incompetent), ignoring the fact that a .0003 millisecond makes no difference to the client's website. It's not Google's fault, but I just had to sigh when I saw some of the rules (I'll expect a call by tomorrow haha). :-)
I could actually see it working in the other direction. Things like YSlow highlight good technique in a (seemingly) objective way.
As for your issue, I would hope that if a client is savvy enough to check with these tools, they could also understand when you tell them what is important and what is not.
I find that my smartest clients have the capability of being the most annoying but usually just demand more explanation out of me. And while sometimes they take that to the extreme (where it's smarter to limit your interaction) its usually for the best because in teaching I find I learn a lot more.
