Our philosophy is, if you can only do a half-Assed version of a song, don't do it at all, as it can make you look "shaky" and kind of ruins the memory of a (hopefully good) song.
That's just us.
I hasten to add that most of the people here are probably better players than me, and thus it's also a lack of instrumental Talent on my part that would more ruin the attempt.
What we DO occasionally try is a Chorus or Verse/Chorus of a song A cappella, with some off the cuff harmonies (a tricky but satisfying thing to do) - this probably isn't the response you were looking for, but it's another view - a Vocalist's view if you know what I mean....