The catch is that trying to tune a piano by ear gets really tricky since your ear wants to tune intervals to integer ratios of frequencies. It's really easy to tune a piano so that it sounds good in C and then the further away from that key you get, the worse the tuning gets. (I've encountered twentieth-century pipe organs that aren't equally tempered—I had been hired to play bass and guitar at a church once and things were fine rehearsing with the piano, but in the church, the guitar sounded horrid and I had to switch to playing bass for all the songs that were accompanied on organ.)
True, hence 'stretch' tuning and various other tunings. It all depends on whether you want the piano to play 'period correct', by itself in a solo concerto in together with other instruments.
There are so many different tunings it is quite amazing.
A great piece of open source software for anybody that is even remotely serious about this:
