A lot of variables, as has been noted above. Depends on the venue, the event, and the focus of the audience.

A song inappropriate to a loud bar venue might be perfect for a library gig or house concert where the audience is not only interested in the song itself, but in hearing the background, history, and scholarship -- and maybe some alternate versions for comparison. (Then again, if it's the sort of gig where your audience is there to drink and socialize, go ahead and sing whatever you want -- they're not listening anyway.)

In a singaround or session, don't hog the limelight and sing your longest ballad -- keep it reasonably short and try to be inclusive by choosing a song with a chorus. Or, in the case of a mixed (vocal and instrumental) session like my weekly Press Room trad session, choose a song with either a good chorus to engage the singers or a good instrumental break to keep the instrumentalists interested...or both.

Be true to the song...but don't bore your listeners.

Linn