Fred Rau at MCN did a story about the 10% rule a few years ago (2006-07) It's not a cure all, but, is the best for all round tire performance, grip, mileage, protection of rims(pot holes) and wearing even.
I've used it for years, having been taught by older riders.
I've always got really good mileage, don't mind riding in rain, and never stretch myself out on public roads.
Hornet 36~40 and KRS 38~41
I rode 22,000 miles this past year. June 1,2010-June 3,2011. Yes, notice I needed 2 more days to hit my goal.
I use June 1st as the start because that was the day I retired in 2007. I've got 58,000 miles since then. I am more concerned with mileage then the last oz of grip.
I use a digital gauge that claims +or- 1% at 100lbs