There are several reasons for practicing as much as you can: 1) You review the theory of the material. 2) You get used to working under pressure. 3)You develop confidence and increase your focus. 4) MOST IMPOARTANTLY, YOU NOTICE PATTERNS THAT ARE REPEATED IN EACH EXAM. There is really a finite number of types of questions they can ask you. If you recognize a question that is similar to a question you've done in a practice problem, you save valuable seconds in testing which approach to answering the question is best. I highly recommend picking up a few new SAT or GMAT review books and focus on learning techniques for each type of problem. Try Kaplan, princeton review, etc. Make sure the book has detailed explanations for each problem. Going carefully over the explanations is key.
If you are pressed for time, though, a focus on QR may not be a good idea. I would say that it is the second hardest section (after VR) to improve your score dramatically as it is not an information based section like TS. Also, for most schools QR is the least important section. Unless, you expect it to dip below 17, it may be more worthwile to focus on the other sections (if pressed for time, of course). Ideally, you should have enough time to study everything thoroughly.
