Aside from Auricks ecxellent advice and synopsis of MatCap. If you want to use ZB default materials I usually bounce back and forth from SimpleShader to FastShader to MatCap RedClay. This helps keep the eye/mind refreshed and gives alternate shadows and specular.
Another technique to use when Retopologizing is to turn the Frame Button on and use FlatShader this lets you see into crevices and small spaces.
One more use of materials is to at certain intervals see how your model looks with many different types of Materials, again this keeps the eye/mind refreshed and the model retains it’s stamina.
Using the Fast shader for ZSpheres seems to be a little more simlpified.
I also like using the AgedBronze shader for ZSphere modeling, I believe that one is in the MatCap library.