Unless there's a freeze potential, keep the water in it. Fresh water will contain minerals, oxygen, and potentially some organic material. Once it sits inside the boiler for a long time, the oxygen is used up, the minerals tend to deposit themselves, and the water becomes more inert. Replacing it with fresh, starts that all over again. I'd check the pH, and if it's neutral, just leave it alone.
Curious what the pros have to say on this, though.