If you notice that your rainfall shower is dripping when not in use, the issue may be caused by air pressure and water surface tension within the can of the shower head. A rainfall shower head contains water on a flat surface and a pocket of air on top, so the water is pushed through the shower head using gravity. When you shut off the valve, pressure against the air pocket is relieved and water is then left in the shower head. The outside air pressure will then hold the water inside. Releasing the surface tension will allow the water to be released.
There are a few methods to try and fix the issue. First, you can slightly tip the shower head after use to allow the water to drain. Another option is to run your hand over the end of the nozzles to break the air tension. The small tilt or tap to the shower head will allow the surface tension of water to break and allow air to flow between the external room and the inside of the rain can.