I always 'filter' through in the same circumstances i.e. if there is a queue and no one goes for the vacant pump. That's there problem and if they approached me I would gladly invite them to use it. I do the same in the car too and will stretch the hose over if need be.
A garage I visited a couple of days ago (in the car) actually told you to use both sides of the pump because the hoses were long enough - thats good enough vindication for me.
