I prefer not ... But it depends a lot on the situation and other factors- like whether it is a 'pubic' venue or a private party ... At a private party, the guest is probably a relative or friend of the host, so it may be difficult to say no ... whatever the situation, I want to at least be able to work out the tune -key, tempo, ending etc. During a break ... The problem always exists that if the 'guest' is BAD, it is going to reflect on the muscian ... and then, no matter what, there is ALWAYS the possibility of opening it up to 'open mic or karaoke night' ...