Put your car on a skid pad to obtain a quantifiable risk from overinflation. I do, definitely, agree that underinflation and overinflation tend to be rather nebulus terms. For a normal driver, without the scientific means to easily quantify over/underinflation (other than the crude chalk method I mentioned), he/she must rely on vehicle feel and observations or go strictly by the placard. A pure hypermiler that is usually under speed limits and takes corners at low speeds may have a tougher time making slip/grip observations. Arbitrarily raising your pressure to max sidewall just doesn't make sense w/o considering other factors. The placard is a "standard" or defines the point from which to deviate from. Typically, the optimal tire pressures for normal driving seem to be somewhere within ±10-15% of placard. For me, I've observed less "grip" or "stiction" on my daily driven vehicle/tires in the summer with pressures any much above 4-5psi above placard and better snow grip in winter at or slightly below placard and adjust them accordingly. I also observed a very noticeable "grip" change when I went from summer performance tires to performance all seasons. You might as well have skis on in the winter than summer/rain tires. BTW, "cold", as technically applied to tires and placard settings, usually means an ambient temperature of 70 degrees and not driven on for 3-4 hours minumum, preferably longer. For ambient temperatures above and below 70 degrees, pressures are decreased/increased by approximately 1psi for every 10 degrees of change. It is more commonly taken to mean first thing in the morning temp on undriven tires.