My own experience would suggest that if a free reed instrument is within 0.2 of a semitone of modern concert pitch then you can play with other instruments without it sounding out of tune. The typical old tuning of A=456Hz being 0.5 of a semitone sharp of modern concert is too far out for the comfort of the listeners.

A multi-voice melodeon will typically have the two (or more) reeds tuned about 0.1 to 0.2 apart.

Scott