However; in a crowded session with limited time and lots of people who want to sing or play, it would seem to be good manners not to choose the longest and slowest song or tune you know (or in many cases, almost know).

And good manners would also be not waiting until your turn has arrived and feigning surprise before only then going to get your guitar out of its case and taking an age tuning it while all the singers just sit around wondering "Will the session end before I get my turn?".

And then playing the longest and slowest song you know - with lots of solo breaks. Have you heard Barbara Allan with an instrumental between EVERY verse? I have. :-(((