This is a rule that most veteran karaoke-goers will agree with. Most of us have been at a karaoke bar and found ourselves listening to the instrumental track, along with a two-beat-delayed vocal track. There is no worse experience.
Before you request a song, ask yourself, and answer honestly: Do you know how this song goes? I don't mean the lyrics, I mean, do you know the configuration? Do you remember where the key change is? Do you remember what parts the singer speeds up, and where they slow down? Do you remember that one chorus where the last line is ever so slightly off-beat?
The answer, my friend, is the same as the answer to the question "Should I request this song?"