When my 2 stroke modding capabilities were in their infancy, I built one of my 044s with something less than .015" squish(I don't remember the exact number.) Anyway, I got it nice and hot, and heard what sounded like a light tapping noise...I tore it down and there was evidence that the piston was just barely touching the squish band at TDC when hot.
Luckily no damage and I've since been fairly conservative on squish numbers. My own saws are usually pretty tight, often below .020", but I occasionally do tear-downs on most of my saws, so I'm not too worried about carbon build up. Other peoples' saws, I'm usually in the .020-.025" range, because most people aren't pulling a motor apart until something breaks.
Speaking of which, most guys' maintenance habits are pretty atrocious. I've seen pistons that didn't have the cir-clips installed(on a dual-transfer Stihl none the less,) foam air filters without oil, and even a couple of saws with holes punched through the piston from metal piston stops.