I was very influenced in my youth by a guy who played in a group with me who was a qualified piano tuner and who passed on a lot of the theory. He was a brilliant about tuning for certain keys and knew how to file reeds on concertinas and other boxes to get them sounding just right. Likewise he was the only person I've ever met that had the wherewithal to drill teensy holes in odd places on tin whistles to match a fixed tune instrument that was not in perfect concert pitch.

Of direct relevance to this thread however, he would always recommend tuning guitar using a piano tuner's approach (for optimum all key performance). This made a lot of sense for a band that played in lots of unusual keys for live performances where there was no time to stuff around between numbers.

I posted similar details some time ago but in essence it involved deliberately tuning the adjacent string harmonics a defined smidgeon out of tune. It needs some practice but in my opinion is marvellous.

Regards, John