Not always full, but we discovered on our first longer trip last year that it tows better with some water in it. Also, like previously mentioned, there's always the risk of the CG you're going to having an issue with their water, or the water isn't that convenient to get to with a larger trailer, etc etc. Nicer to be able to just pull in and not have to go find water first thing straight off (most places we go do not have water at each site).
This summer we're heading up and over I-70 through the mountains so we probably won't fill up before we go, but we'll have some in there just to help with the smoothness of the ride.
