The tricky thing with the "debate" on waiting at the cashier versus going directly to the counter is the perception of the people who are waiting at the cashier and the people who are waiting in the lines to pick up their food. Those people don't know the person(s) going "around" them are only going for water. It doesn't make sense to wait in a long line at a cashier if only getting free ice water but then again, if you go around those people, then you feel badly. It's kind of a toss up.
Personally, I just use the water fountains.