The farmer I worked for years ago said the same thing. He actually built a shed out in his fields with a huge sink hooked up to the well so that he could get the veggies washed as quickly as possible.
I find it definitely makes a difference with things like tomatoes, which can over-ripen easily even after they're harvested. And a wash with good, cold water (I add ice in summer, since my city water is usually around 80 degrees when it's hot outside) will definitely reduce wilting of fresh greens. I don't pre-wash anything I'll be storing at room temperature (potatoes, onions, melons, winter squash), because I think it encourages fungus growth.
