If groups disagree about an answer, that's a perfect opportunity to have them present to the class. Nevertheless, part of math is correct answers. I would claim, however, that students in school have plenty of opportunities to know for sure whether they're right or wrong. One teacher decided to let students make an answer key. If there's no answer already, the group writes one up and puts it in the book.
Students are more likely to question answers their colleagues made up.
I will choose some solutions to post on the web so we can have a world-wide answer book for United We Solve . Some brave souls have already done so.