I believe that most of the time you should not show the histogram after the first, individual poll. The only exception would be if a overwhelming majority got the questions right and the instructor wanted to have closure quickly and move on.
I suspect that people who answered "no" to this are thinking that showing the histogram may bias the students' thinking during the discussion, making them lean toward the most common answer without having thought things through (if there are other reasons, I would be interested in hearing them!). I recognize that danger however I suspect that showing them the spread of answers may benefit both the over-confident and under-confident students, and also help convince the students of the benefits of Peer Instruction. To wit, 1) those who were convinced that their knee-jerk answer was correct without really having thought things through carefully may realize upon seeing the histogram that perhaps there's more to it and that they should slow down and think a little before "picking an answer"; 2) seeing the variety of answers may be helpful for those students who think that they must be dumber than everyone else if they don't see the correct answer right away, it may help them get over their self-defeating attitude; 3) in a group that is not yet convinced of the value of participating in Peer Instruction, seeing the before and after histograms provides good ammunition when trying to win them over. Admittedly, these are only impressions and opinions, I don't have any data to back this up. (And, when using flash cards, it's rather a moot point anyway!)
Unfortunately, not all clicker systems currently in use (for example, Turning Technologies' Turning Point Anywhere for the Mac) allow the histogram to be hidden. I'm personally trying to get that feature introduced in the newer version of the Anywhere software that's currently in beta form and should be launched sometime in May(?).
-- Phil S.