I used to be somewhat serious about using the same pump to the point that I would actually wait for it to be free if someone was there.
One time I got to the station late at night and one other car was at the pump I prefer. So I parked, bought something from the convenient store, and got back in my vehicle. As soon as I started up, somebody else pulled into the station at whatever obscure hour of the night it was...and went to the same pump.
These days I'm less particular. I figured that if the goal is to use less fuel, then idling or restarting the engine to get a certain pump - while perhaps good for calculations - isn't worth it. If I can get the same pump I will, but I'll take whatever is free and let the average work itself out.