I have some sympathy with Dave, although I wouldn't go as far as staying away.
I think humorous alterations to songs, or the singing of parodies, can alter the tone of a singaround, usually, in my opinion, for the worse. By that I mean that it is usually a signal that the evening has moved away from straightforward renditions, and, instead, there will be one "humorous" song after another. It is a hard road back, after that.
Nevertheless, most people seem to enjoy them, so who is to say what is right or wrong. It could even be regarded as an essential ingredient of a successful evening, by many, especially after a few pints!