As someone who has sailed in saltwater far more (and longer) than in freshwater, I have to admit, I do minimal boat washing (once a year unless it gets really dirty). Probably not the best practice, but I haven't noticed any ill effects.

Now, go to a weeklong event on saltwater, with freshwater sailors in tow, and hoses available, and you will see people washing down their boats and sails EVERY DAY...

Mike