[geofft]

More precisely, modern keyed instruments are slightly off tune. :) I'm pretty sure unaccompanied voices (either solo or in groups) tend naturally towards just intonation in the current key, because you don't think of it so much as "E to G#," you think of it as "up a major third." I'd sort of assume that players of continuous-pitch instruments like violins and trombones also do that instinctively, but I only have firsthand experience with singing.

[matheist]

I play piano, guitar (classical music), and trumpet (jazz).

The guitar is aspirationally tuned in equal temperament — note that one can transpose a piece up by playing in a higher position and it ought to sound the same. In practice, though, I would find myself often bending notes to make them sound better (closer to just), especially major 3rds. Sometimes depending on the key of the piece I was playing, I might even tune a string slightly off so that the major 3rd would sound better. E.g. when playing a piece in D, I might tune the top E string so that the F# (the major 3rd) on the second fret sounded good.

The trumpet is definitely not tuned to equal temperament — but it's sort-of "continuous-pitch" because the player can bend ("lip") notes slightly flat or sharp, depending on the skill of the player and the properties of the instrument and mouthpiece. In practice, I'd find myself lipping to a greater or lesser extend depending on the key I was playing in, which has more to do with the trumpet's physical limitations (it's impossible to actually make it perfectly in tune in all keys) than the details of which tuning system was chosen. Once I know I have to lip notes into tune anyway, I choose the tuning based on what sounds good, rather than aiming for 2^(7/12) or whatever.

Finally I'll note that even the piano, the prototypical example of an equal-temperament instrument, is not tuned according to equal temperament, because the vibrating strings' stiffness means that the overtones are not harmonic. (They're slightly sharp relative to harmonic.) That means that for a piano to sound in tune with itself, pitches are slightly stretched (flat at the low end, sharp at the high end).

[mhauru]

Agreed. Although as you point out, the singers being unaccompanied is important. Mixing keyed (or fretted, valved etc.) instruments into the ensemble quickly starts to push everyone towards 12-EDO.

I also wonder how much continuous pitch instruments do this, when playing pieces of music and with ensembles that would allow adjusting to just intonation. If I walk up to a good violinist and ask them to play a simple melody on a single string, do they gravitate towards just intonation, or does muscle memory make them place their fingers for 12-EDO? What about a superbly good violinist?

[genericguy]

I'm a violinist! The answer is "it depends". There are so many complexities to tuning that relies on harmony which is continuously changing. Mostly it is done with careful listening and intuition. Even playing with a piano, we won't play in equal temperament 100% of the time.