Remember, there's a difference between the yeast lasting for 6months or more and being alive.... And the yeast being fine to just pitch into a beer.
You can store yeast under sterile water or beer and it will stay alive, but that doesn't mean the cup of slurry you washed and shoved in the fridge 8 months ago is fine to just swirl up and pitch. Yeast starts going down hill from a viability and vitality perspective immediately, and drops off to quite low levels really quite quickly. You cant expect more from your washed yeast than you would get from a wyeast pack or whitelabs vial. So assume the same sort of viability that say the Mr Malty calculator would give you for a pack manufactured on the same day you washed your yeast, then chop that viability in half because you aren't as good as wyeast, and your viability from a washed Cake would have been lower anyway.
If you wash a yeast cake and intend to re-pitch it directly, do it within a week or two, otherwise you should be making a starter out of it anyway, so you might as well just save a small amount.
You want to store under sterile water, not distilled, distilled will pull salts out of the yeast cells, so will RO.