I do about twice a year.
Having Vancouvers, there is enough room to get my mitt between the spokes and wash the insides of them as part of my normal wash regime. This won't get rid of tar spots, to they need to come off to be able to clean them off. Whist the wheels are off, it gives access to the parts of the wheel arch liners that can't be reached with the wheels on the car. It's also better to polish and seal the wheels when they are up on the bench, rather than crawling round on the floor.