Below is a method to establish a usable tooth to a drawing surface that will no longer accept material due to the diminishment, or “burnishing,” of the paper’s tooth. Paper used: Canson Mi-Teintes Drawing Paper (98lb press.) I wouldn’t recommend this for anything thinner.
There is also a video of a more subtle fix that does not involve removing material first here:
Good question. The early line exercises and pressure scales use both as do the repetitions. However, Gradation Block, Patterns, and Form Boxes are all recommended on the smoother side.