In my opinion, re-reading takes up a lot of time and should only be used on tough questions that are difficult to understand. I would recommend taking notes while you read. That really helped me understand the questions better as I was reading them. This goes for math questions as well.

Tricky question! Truthfully, it comes down to what you do/don't understand. Remember, your job on the GMAT is to answer the question(s) asked. If you don't understand what you need to know in order to do that, you have no choice - you either re-read or you guess.

In other cases, though, there is information given that you don't need in order to answer the question. Examples include:

-Background information in Critical Reasoning passages
-Pure content in Sentence Correction questions
-Details in Reading Comprehension passages (these tend to be huge hangups - wait to understand details until they ask for them)

The key is to read proactively. Know what you're being asked (or might be asked on RC passages...usually it's more about the author's intent than about any pure details) and look for the information you need to know in order to answer. Don't get hung up on details in an RC passage and re-read to try to learn about the topic; but if you don't know what the author is trying to do in that paragraph (introduce a theory? Criticize an an argument?) you need to re-read.