I find it depends on you, your choir, your room, etc.

Last time I conducted chant, it was with paid singers, and I used a pitch pipe (although for the first few months the group was together, I would actually play the first few notes of the incipit on the organ).

When I conduct my undergraduate men's choir (who perform concerts, not in liturgy) made up of mostly non-majors, I give all pitches from the piano.

I'm capable of using a tuning fork, but have never trusted myself or my choirs enough to have an opportunity to do this enough.

There's no one right way. Many people find that a piano is not helpful to learning chant. I agree in principle, but maybe not with the same vociferousness. If there's one handy in your rehearsal space, it may be helpful to use to check reference pitches every once in a while to keep your singers from accidentally "getting used to" sinking in pitch.