I generally support good manners. I try to be curteous in most situations (as with everyone, I certainly lapse now and again). Some times, however, that "polite gesture" is just a pain in the rear for everyone else around you.
Mornings lately I've been taking the complex shuttle (ei ride?), and have been fairly aggravated by this one fellow's consistent behavior. Typically a line forms up for entry. He is in line, and stops at the door and offers people to get on before him. His gallantry delays everyone just that little bit, for no discernable reason: DUDE, you are in a line, are you unaware of the function?
As ever, I think the best rule is to try and act to minimize the hassle (technically defined as the product of the time to accomplish the task with the sum of WTFs and "No you first"s [Interesting: hassle has units of time. That seems right, actually.]) for all parties. The result is that when everyone else is clearly operating by the "first come, first served" rule, your adoption of the polite persona imposes a cost on everyone else. Thanks!