I'm firmly in the DON'T join in the verses. By all means in the chorus.

When I'm singing a verse I have my own timing which may not be matched by someone else, my own emphasis on words, which again may not matched by someone else.

Unless I have practiced a song with someone else I find it is distracting to say the least to have someone else singing along with the verse.

One final word, if joining in with the chorus take the cue from the singer not from your own interpretation.