I use high heat most of the time with no problem, I concider high heat to be hot anough to scold you if you touch the water.
I turn the temp' down when cleaning dralon or other fabrics that have a pile but all other fabrics get the full temp'
on carpets I always use the full temp' even on wool, I've heard that it can untwist the fibres on some carpets but I've never seen this.
I speak to alot of carpet cleaners who are weary of using too much heat but will blast a carpet with a high Ph chemicals, which to me is much more risky
Mike