I work a lot with singer songwriters, some just don't like using a click track and that is actually fine. The music might be better for it. So it depends on the song and where it's headed. My rule is if there's going to be drums then you have to play to a click track. I tell them it will take way longer to work on the song with out a time base and this will cost them money.
But Folk music is not normally drum music, so there should be no problem overdubbing other instruments with out a click track.
I think you stand a chance of trashing the feel if you go down that road with certain songs.
Now if the original artist had real, real bad timing then that does make it a challenge. Hopefully they played solid and changed chords on the beat. Speeding up and slowing down is not a real issue, but slopping timing is. I'll make them re do the song. Or I'll play along with them.
I usually start with laying down a bass part before there is to much clutter.
I know not everyone can work this way but it's just a matter of listening very closely as you play along. If some of the musicians are struggling without a click track ask them to at least try. Might be the snow ball effect,however, bad timing on the bed track, and session players with bad timing too. Good session players will make a difference.
But to answer your question:
I use the above mentioned method of playing along with a hi hat and then fix any off "sounding" hits. I don't even try and match Sonars tempo and measure grid unless later tracks are to be quantisized. Then you spend hours fussing with audio snap.