I always tell my students that to me, a dirty test is a beautiful test. Marking up reading passages and math questions help to focus your brain on what is important and what isn't. I cannot tell you how many times I have seen students make mistakes because they missed a part of what was being asked of them. This mistake is easily fixed by boxing in or underlining the important words in each question.
The reason I use the term "dirty," is to remind students not to leave their test pages blank. This does not mean that their markups should be sloppy and messy. There should be marks and notes all over each test page, but those notes must be clear enough to easily read. This is especially true in the math section. Sloppy numbers are incredibly dangerous. Math is exact, so if you accidentally calculate a 6 instead of a 0, the answer will be wrong.
In sum, a dirty test (not a messy test), is key to success.