Concerning whether to take another exam. It is a tough call, but a few things to take into account:
Schools do not normally care if a student takes the exam multiple times. That's ok. Can one apply with less than a 700 or a bit less than the school may ask for? Of course! The application to an MBA or other program will depend on several factors, including the GMAT. Now if a student gets a 350 on the GMAT and needs roughly a 700 then of course a retake is in order but if schools are asking for a 700 and someone gets a 690, I would still say go for it, not because a 690 is not excellent but because if one can get a 690 then when can certainly even jump to a higher level. In fact from one exam to the next scores can vary wildly sometimes. A score can dip from one exam to another even with the same amount of knowledge or the same percentage of correct/incorrect. The GMAT scoring algorithm takes many factors into account including where you make errors (eg making mistakes on 'graded' versus experimental questions, or the 'weight' of the question itself etc), how many mistakes you commit in a row, whether you finish both sections or leave questions out, whether the last few questions of either section seemed like they were guessed at (ie did you spend only a few seconds on the last few questions as opposed to the full time you should have), how you felt on test day - these can all affect score.
From one test to the next you can easily jump +/- 30 points (and even up to 80 or so) depending on where the mistakes are. So you can get the same percentage of right and wrong on two exams with the same amount of knowledge and yet have a score that differs by quite a bit. This is the same as the real GMAT. To illustrate, I can make a mistake every other question in the Quant over 5 exams (ie in every exam my percentage is 50% but have a score range from, say, a 15 to a 35, for example, which is quite insane, but it shows too that where you make mistakes can often be more important than how many. Overall, when a student wants to know where he/she stands, the best way to do so is to take the average after a few exams. This should get you to a fairly close estimation of where you are at. You can also check out our blog for more info on GMAT scoring (gmat.economist
I do wish to add the following. While I like to encourage students to retake exams if they have the energy and time to do so, in certain circumstances in may not be necessary especially in the context of other application criteria. So altogether a retake is good for students who are both far and close to their GMAT range but this will take dedication and further practice. If you have it in you to do so then go for it! If you have other aspects you need to take care of but are already at your range then I do not see it as all important. The other aspects of the application may carry quite a bit more weight. And finally, you can always ask the school itself whether it is worth doing. Sometimes they actually help in that regard.
I hope that helps.
Economist GMAT Tutor