Originally Posted by WhyFi

Uh, one of those quotes is completely unqualified (not explaining why they replace as soon as it starts to show signs of squaring off) and another isn't addressing wear per se, but rather the suggestion of putting a slightly squared-off tire on the front wheel in an effort to balance wear (yeah, I wouldn't do that, either).

Not only are the wear indicators there for a reason, but you can bet that they're a slightly conservative gauge. In terms of wear, unless the tire is getting beat up enough to make it significantly more flat prone, there's no reason to not ride to the bottom of the indicators, at the very least. After that, some are comfortable to continue riding, keeping a close eye on the tread, until punctures threaten to become problematic or cords start showing through.