With regards to tunes - unless I know a piece of music really well, the dots are a useful guide as to what I am playing as it is not always easy to pick out the melody in a session,but after that I tend to play by ear as I find following the dots a bit pedestrian.

With regards songs, I always refer back to the dots if they are available, as I feel a lot a the original nuances of the original tune can be lost when it has been passed on repeatedly by ear, particularly in the case of shanties.