I'm not an expert on this (we have soft water) but I'd say even if you used the hard water, as long as you don't let it dry on the finish it would be fine.

There's nothing saying the gas station/car wash place would have a softener either. Using hard water would cause you to use more soap, but I'm sure you know this already.
Judging by his attitude (beckoning with finger?!?!?

) I'd say this guy was stroking his ego under the guise of "helping". I hate that.
btw, was this guy referring to places with a water softener, or was he under the impression that these places get this magical "special" soft water supplied exclusively to them? LOL.