More like completely fucking necessary on the back of white cars.
Another thing I do is have a credit card (or something else sharp plastic) handy, if I see a spot is being stubborn I’ll take the edge of the credit card and agitate JUST the stain a little bit. I’m talking about like freaking microscopically (not really), but like a 1mm area. I don’t rub the cc along the panel. It helps break the shell of the stain and the clay will remove it much more easily.