I've never washed a sail in my life and they last just fine.
I think this sail washing thing is a leftover from the old canvas sails of a million years ago. If you didn't wash and dry them then the next season they would be a nice shade of black from all the mildew on them and be half rotten.
That doesn't happen with the new materials so instead of wasting time with the washing, stay out another 15 minutes, and when you come in, roll them up still dripping wet and chuck them in the back on an old towel or two maybe to catch the drips.

This will make you feel much happier unless you have a washing O.C.D.

{edit} I should have added however, this does not apply to kites and ALL kites need at least a 1 hour wash and gentle rub down afterwards to maintain them in a flyable condition. Make sure that this routine is rigorously maintained. Also a 90 minute rubdown and line inspection prior to each use is also recommended. {/edit}