About pianos: how in-tune the piano is depends on when it was last tuned. One pays to have them tuned, then they slip until it simply can't be endured, then one tunes them again. How a piano sounds against your guitar depends on where the piano is in that cycle.

There is another factor: for example, my church has an old (1950's, maybe) Steinway grand. We used to have a violinist with an excellent ear. (She has a master's degree in violin performance.) She told me that the piano changes tuning throughout the service, and that she listened to it and adapted her playing to match it as the service went along. Not to be outdone, I tried to match the tuning of my recorder to her violin. (The piano produced too many notes for me to try to match it.)

Recorder, flute and violin are both instruments that you can adjust as you play. Piano - not so much. :)

Whether you call them violinists or fiddlers, fiddlers' notes can vary a lot. Fiddlers should be listening to the group and making sure they are in tune. This is not complicated; just listen and ask yourself, "Does this sound good?" If the guitar is a little funky, a fiddler can adjust his notes to match it.   

I remember a country dance where the fiddler played out of tune all night. When we got done, I felt like somebody had brushed my brain with a suede brush. Nobody said a word, probably because all we would get is snapped at.

(When we are getting a band together, I "call in sick" if we wind up with two fiddles.)