It makes rendering a lot easier having waterproofer in the scratch coat, especially if you're working on a high suction background. I "always" use it. Try top coat rendering onto a scratch coat without w/proofer in it,,, not easy.
But,,,,
"Don't" use waterproofer in the top coat if you're going to tyrolean finish onto the render,, and "don't" use waterproofer in the scratch coat, if you're going to wet dash,,, you need some suction for both applications.
Roughcaster.