Mental processing vs. using scratch pad

Depends on how much you can visualize. For quant, I find it easier to write, except in DS questions where you can visualize a lot (but make more mistakes). So, its kind of a trade off. Do some mock tests or time bound practice and see if you are able to solve all the questions within time. If yes, no need to change anything, otherwise try to visualize more in the DS questions, where you dont really need to solve the question in many cases.
For verbal, I never wrote anything down other than crossing off answer choices. Even though MGMAT suggested taking notes for RC and CR, I found that I ended up reading the passage again anyway. So, again, do some practice and see if you really need to write stuff down for verbal.