Tuning a piano by ear. There is no better way.

"Why does Jurgen tune pianos by ear?  Simply put, because he can. Tuning aurally (by ear) is the time-honoured way to tune pianos.  Aural tuning involves listening closely to the individual strings, the unisons, and intervals such as octaves, thirds, fourths, fifths etc.  The piano is a polyphonic instrument, meaning that numerous notes are made to sound simultaneously.  All these simultaneous notes interact and have to blend in specific ways to create lush tone colours and dynamics.  In an aural tuning, these intervals are used to dial in the perfect pitch of every note, according to how it sounds in conjunction with others.  For Jurgen, it makes little sense to tune individual notes outside of a musical context with each other, which is what an electronic tuner does. While it is possible to achieve a good tuning with modern high-tech aids, all too often the 'black box' machines are used as crutches by those who did not benefit from the extensive training required to learn the skill set required for aural tuning. In the end, the human ear must be the final judge, so why not let ear direct the process from the start? Most of my distinguishing clients – professional musicians and teachers – appreciate the method and care I use to tune their instrument and prefer my work because I tune aurally."

Excerpt from:

Piano Forte Service and Piano Tuning