This is indeed a myth. People who say this do not understand how computer adaptive testing (CAT) works. They're trying to apply their "intuitive" knowledge, but our intuitive testing knowledge is based on taking non-adaptive paper tests—a very different thing. We have reviewed the academic books, papers, and patents on CAT, and we have discussed this extensively with GMAC. It's a myth.
The earlier questions are not worth more than the later ones. The only questions that are worth slightly
less, in the end, are the couple of easiest and couple of hardest questions that you face throughout a section. They are discounted as "outliers" in your performance, particularly if your performance on those questions was at odds with your overall performance. In other words, if you get a couple of easy ones wrong but, overall, your performance is much higher, then those 1-2 questions won't count against you as much. On the flip side, if you get a couple of very hard ones right, but your overall performance is much lower, then those 1-2 questions won't contribute to lifting your score as much.
We had a teacher once go into the real thing and get the first 7 questions wrong in a row. (She didn't just guess. She solved to make sure she was choosing incorrect answers.) Then, she took the rest of the section normally. She usually scores a perfect 51 on quant; she scored a 48 or 49 (I forget now). So she was able to recover almost completely. And it's not surprising that she didn't get all the way to 51—after all, she did get at least 7 questions wrong, a lot of which were lower level, by definition. You can't miss many easier questions and still score a 51, regardless of where those 7 questions are located.