It's really not ok to store it, it says it on the basin itself. There are little cracks that still hold water inside the basin, unless you left it off for a day or over night then I would say it might be ok. It can lead to bad smells and bearing damage to the motor possibly.
Don't know if anyone mentioned but you should change the water often while cleaning unless you have a really clean house. It helps cut down on the fine dust that can clog the hepa and coat the fan blades.
Colby