I agree with Ron on this one. Using tank water is better in a lot of ways: the risk to your boat is FAR less, your FW system stays maintained because you are using it all the time, and the water in the tank stays cleaner if it turns over frequently. I've expressed my views on DRINKING tank water many times. I think it's a bad practice as it's not clean enough for consumption. But for showering, dishes, etc, it's fine.
Even a well-maintained FW system isn't designed to operate at the pressures some dockside systems operate at. Not betting your boat's life on the integrity of all those connections in the FW system, no matter how well you keep it up, makes sense to me. The dockside system is fine for filling your water tank. I usually keep mind at about half full. That's only 35 gallons, more or less, so, not a lot of weight to haul around.
When I connect to dockside water, I use a valve on the boat and the faucet control on the dock. If I am not on the boat, I shut both off. May seem like overkill, but I don't need my boat to sink from a fresh water deluge that I could have prevented.