I have difficulty in starting a song when there are people still fiddling around with instruments (and Manitas is one who should know better!!!), which is why I didn't sing at the Jug back in June..... If you are in an accepted mixed session, then the musicians should have the manners to not play whilst others are singing, after all, we don't sing whilst they are playing, and I know plenty of 'tunes' that have words to them!!If you are new to a session then a few discreet questions or just listening for the first 20 - 30 mins should give you an idea if songs are acceptable, tolerated or ignored.
LTS