Is it crazy to leave water in your tanks in storage?

There's no need for the gasket material to be kept moist with water. If you think about it, the seals on the output side of the valve (which are the same material as on the input side) are dry practically all the time, or at least should be. The whole point of the gasket, be it rubber or silicone or whatever material they use, is to be impervious to water as it seals.