I have especially dirty tap water (small particulates), though I'm not in a very hard water area. I filter all water for particulates when making up developer, stop and fixer for films using a Paterson water filter over the tap, but never bother with distileld water as I don't have a ready source. I wash films again using filtered water then do a final rinse in distilled water but emptying the tank, letting it drip for a while then using about 300ml of distilled water, give it a really good shake, then empty, then repeat and add a little wetting agent. Before this final distilled water wash I usually had a few small drying marks, not I haven't seen one for ages. One final tip I picked up somewhere is if you are getting drying marks, when you hand the film up to dry, angle it by attaching the bottom end to a wall or something. That way, as water runs off the neg, it runs to the edge where if it leaves a drying mark it doesn't matter. If you leave it vertical, the water just runs further down the neg then dries. Since using distilled water for the final wash I've never had to do this as it leaves no mark.