GMAT Strategy

I normally recommend that people do one of the official GMATPrep diagnostic tests (from mba.com) very early in their preparations- soon after you familiarize yourself with each question type (particularly Data Sufficiency questions, which can seem mysterious the first time you see them).

Doing a diagnostic can be very helpful for a few reasons. Among others:

-it puts all your learning in context. You know what kinds of questions to expect on the test, and how the concepts you're learning may be tested;

-it does help you to discover your weak and strong points. If one of your scores (Verbal/Quant) is much lower than the other, you will learn where to focus your time while studying. That said, do invest some time preparing for both sections, regardless;

-many find GMAT math much trickier than they anticipate. You'll find out if that's true for you;

-you'll discover whether you need to work on speed or on conceptual understanding (or both);

-it will give you an idea of how much work you'll need to invest to achieve your target score.

All of that said, do *not* become discouraged by a low score. With good preparation, you'll make significant improvements. And, as you point out, it can be very rewarding to see your improvement by doing diagnostic tests periodically. Because the GMATPrep tests are computer adaptive, you'll see harder (i.e. new) questions if you improve, so you can take the same test more than once.

This is just my opinion, of course; hopefully other GMAT teachers and test-takers will offer advice as well, and you can make a decision based on the input of several people with GMAT prep experience.