I didn't realize until now that there was such a thing as "English sessions." Is this the tradition to call them "sessions," or are they called that after Irish sessions?
Over here, sessions are sessions - ITM doesn't have copyright on the term. We have Irish sessions, English sessions (contrary to widespread belief, there is such a thing as English music), Scottish, Welsh, French, Klezmer, Scandinavian, Bluegrass and pretty much anything else you can think of. We also have mixed sessions where anything goes.
I've noticed a frequent assumption on concertina.net that traditional music means Irish music. Perhaps the OP made the same assumption - he didn't specify Irish music in his original question, but many of the replies assumed that. However, many of us play other types of music (and I often play Irish, but not on concertina) and many of the responses reflected that, and pointed out that in these sessions the culture and expectations are often quite different from many Irish sessions
I happen to think that it muddies up the music if people are "harmonizing" on tunes they don't know. According to my observations this is consistent with the opinions of most of the Irish players at sessions I've been to.
I think that on the whole the response to this thread has been broadly in agreement, so far as Irish music is concerned. Irish music emphasises the melody, and there seems to be a widespread feeling that harmonising is not part of this tradition. Whether it "muddies up" the music depends on the skill of the musicians - the Chieftains seem to get away with it pretty well - but it's also a matter of preference, not just of the individual but of the collective session.
English sessions are quite different. They tend to be dominated by melodeons (and English melodeon, unlike Irish accordeon, makes full use of the chord buttons), and concertinas played chordal-style, so there is already a lot of harmony going on. The emphasis is on rhythm, and it is expected that in the course of a tune we will play around with rhythm, chords and counter-melodies. Does it get muddied up? Well, to be honest, quite often yes, but when it comes together its fabulous.
But when you visit someone else's session please be sure everyone's into the the whole harmonizing thing before you start going crazy. That's all I'm saying.
Absolutely. Whether its because of "dogma", a view of the tradition, of just a common preference, it's always important to get understand and respect the culture when you visit a session.