If you haven't shown in a showmanship class in years, then you may not be up to the current trends. In which case, there isn't much reason in arguing with people who are currently competing in showmanship at the breed shows.
Looking back over your shoulder should not be done. Looking at the judge as s/he dismisses you from inspection is fine, but you should never look back while leaving a pattern. If you would like to see this added to the AQHA rulebook, feel free to submit a rule change proposal.
Coming in a month late to this discussion, but I show AQHA and I NEVER look back at the judge. Once the pattern has ended, the judge is scoring you and the NEXT exhibitor is already working the pattern. Ask any trainer that specializes in SMS and they will tell you the same...looking back creates unstraightness problems. And what it says to the JUDGE...is that you are not confident enough to get a straight line, hence you are checking to see if you are straight.