Your "GMAT transcript" will report only your three most recent scores. So if you took the test four times and scored 450 each time, but didn't have your scores forwarded, you could still pull off that miraculous 700 the fifth time and then send your scores. The admissions office would see only two 450's and a 700.
It doesn't really matter, though, whether you send your transcript after each test or wait until you score the number you want. In the end, the school's application will ask you to specify the score you want the admissions committee to consider.
What About the Essay Score on the GMAT?
You will have to write two essays on the GMAT (the "Analytical Writing Section"). They will be scored on a scale of 1 to 6, and those points will not be added to your other GMAT score.
The essays are extremely simple. I teach my students a basic template to follow and they seem to do very well with it. They regularly score perfect 6's (the 99th percentile) just by following the template.
Your essay score is pretty much worthless, so I spend very little time on it in class. A few years ago I asked the admissions director of a Top-5 school what she was doing with the essay scores. She laughed at me and said she wasn't really using them. I have to agree with her. The GMAT essays are ridiculous. Given how well some students have done by just following a formula, the scores seem pretty meaningless. And now the essays are being graded by computer, making them even more worthless. So don't spend a lot of time studying for the AWA essays, and don't put too much emphasis on your essay score. I can assure you the admissions committee won't.
Having the list tempts some people to try to write their essays ahead of time. The pool of essay topics is so large, though, that it would be impossible to write an essay for each (and remember them all) before test day. Your time would be better spent studying the other sections of the GMAT.
Should I Take a GMAT-Prep Course?
If you hope to go to a top school, you'd be crazy not to prepare for the GMAT. Being admitted to Kellogg or Columbia or a similar school is well worth the time and money invested in a good prep course.
I don't want to steer you toward one company over another, but I would suggest that you take the longest, most comprehensive course available in your area. And look for a good instructor. A good teacher can reveal subtleties about the test that aren't written into any book, and just having the structure of a class will force you to work harder than you would if you chose to study on your own.