I know what you mean. Believe it or not, often some of the people at the bar who don't appear to be listening really are. I've played sets in pubs/bars where I was convinced nobody was paying the least attention, and had people come up afterwards to tell me how much they'd enjoyed the set. You have to just use your judgement as far as including familiar chestnuts on your set list but in general I think unless a song is really, really catchy, if it isn't 'familiar' they tune it out. It is very difficult to play in bars, unless you're somebody who really knows how to 'work a room' and that takes a lot of practice and a certain 'knack'.