I rinse it out until the water runs clear. I've found, over years, that if I don't get soap, cream or croap out after every use, there will eventually be a build-up of hardened soap in the core of the knot. I have always thought that a brush in good order should splay from tip to handle, and a brush with a hard core of soap residue won't.
It's probably a lesser concern with synths and cheaper to replace brushes, but matters with any brush, especially badger.