My Sugino messenger does the tight/loose thing you talk about. I just put it on. I'm not sure if it's the chainring, or the crank that's off kilter. That kind of stuff bugs the crap out of me. When I get home from vacation I'm going to pull the chainring off and do some precision measuring (kind of like how you true a rim) and see what's off.

Here's why it bothers me. The chain is either getting more tight, or more loose. On the "getting tighter" 180 degrees of rotation, the chain likes to vibrate and at certain slower speeds can be kind of annoying, but only because I'm super anal about that stuff. Most people I don't think, would notice.