There's no fixed rule about this; chant is written in a movable-Do system and the pitch can be adjusted according to the group. Some pieces with a broad range make the choice of pitch not so easy.

As Chonak points out, pitch in chant is purely arbitrary, depending on the singers' range and the chant's tessitura. It is, though, your choirmaster's (very eccentric) prerogative to insist that all reciting tones (in his or her jurisdiction) always be A.

And yet, MJO, you are really suggesting similar criteria without the specificity of a particular pitch. The "reciting tone"; the "dominant"; the "tessitura" are all looking at the same thing - where will my singers be hanging out pitch-wise for the better part of this piece?

Nearly always, I try to pitch the piece based on the voice of the cantor(s). For the lower voices, that typically puts the dominant around A... sometimes a half-step higher or lower, but typically around A. For higher-voiced cantors, I typically find that raising the pitch from 2-3 half-steps works well which would correspondingly raise the dominant. I base it on the cantors because typically the cantors sing the verse (Gradual / Alleluia / Tract / Introit / sometimes Communion or Offertory) and in a sense are most likely to manage the piece if adjustments need to be made - I would prefer they are most comfortable with the range.

Just my 2 cents...

Actually, I usually take a pitch from 'thin air' after consideration of the tessitura and mood of the chant. I also recommend that my cantors do the same. But then, we don't (and never will!) accompany our chant. Only for those who like their chant accompanied does the necessity for a specific pitch relative to an instrument and our A=440 system arise. (Nor, I should think, would it have arisen for our chant ancestors.) Too, I have absolute pitch (though it is not quite as absolute as it used to be), so having to rely on a tuning fork, pitch pipe, or a tacky organ 'note', is not a necessity for me.

It depends on who is singing. If the whole choir is singing, I tend to have Do at A (or A flat if earlier in the morning); if only the Basses, maybe G; if only the Sopranos maybe B flat or B natural; if it's myself cantoring alone on a weekday (I'm a tenor, and prefer singing higher than lower), then I usually use C as Do. But, all that can change based on the piece, time of day, who is sick, etc. I very often will start rehearsing in A, then, if I don't like the way it sounds, I move it up or down. It can even depend on the room: my church favors the sharp end of things in C, G, D, ...

Also, if I really want to do an alternatim organ performance, and Frescobaldi (or whomever) has put mode VII in F with one flat (making G the dominant) then we will sing in that key--even if otherwise I would have moved the dominant up or down in unaccompanied performance.

The only hard and fast rule is to put it in the key that is most accessible for your singers--it just so happens that often this is A.

Too, I think that when learning chant (which is probably the best way to begin learning music, period!) a consistent A Tenor/Reciting-Tone/Dominant as a reference pitch is good because it can train the ear to recognize that pitch. I don't have perfect pitch, as such, but I can usually pick an A out of thin air after singing chant for so long.