I have only cleaned my twice but each time I did soak them in simple green and water. Pool was new and a lot of plaster dust in the filter.

A good indicator would be that the pressure does not drop all the way after cleaning. At that point a soak may be necessary. Also, if your family uses a lot of sun screen, I would suggest a soak each cleaning.

I am planning on only cleaning mine twice a year so it is not much extra work to soak them overnight between filter cycles.

I dump the water near the pad where I have just dirt and bark and a low spot where it collects and then is absorbed into the dirt. I know of others who clean their filters on their lawn and dump the water there.