I always go ahead and just put the plastic or wooden bar up, even if no one is behind me.
I don't really get too upset about it, but a couple of months ago, I happened to go through the self check out lane - scanned my stuff, paid and proceeded to the end to bag my stuff, this guy comes up and started scanning his stuffand sending id down the belt. Meanwhile, I was trying to bag my few items as quickly as possible, but here is this stuff coming down the belt at me. I was miffed, to say the least. Finally, I looked up at him and said would it kill you to wait until I was done? He just looked at me like I was from Mars.
I also like to have all the refrigerated items together and so on - like items like pet food, cleaning items I also like to keep together, but I am crazy and OCD and I know it.
I can handle putting the bar up in front or behind, it is the stinking rude people that drive me bananas, you know the ones, the ones that act like they are the only ones that count and you should be getting out of their way.