I didn't know that it was possible to leave one blank. You should definitely guess if you're not sure and move on.
My biggest break-through in preparing for the GMAT was the realization that sometimes guessing is a good strategy. Let's say you have a difficult question in front of you, you spend ten second reading it and you think to yourself, "This is really hard, it might take me three or four minutes to figure this out, but even then I might not figure it out." If you know you have a really hard one in front of you, you can either take a gamble and spend a lot of time to MAYBE figure it out, or you can just guess and move on. I did this a lot on the test and I got a 750. By doing this, you skip a hard question and save yourself like three or four minutes that you would have wasted, and if the guess turns out to be wrong, the next question you get will be a little bit easier. So then as you continue and as you get questions right you will work your way back up to hard questions, and the next time you get a hard one, you can think, "Hm, this could take a while, BUT, I have plenty of time to work with." or you can think, "Hm, i should skip this, it will just take too much time."
So that was my approach to time management, if I had a really difficult math problem in front of me, or a sentence correction that just didn't make any sense, I would just guess and move on. I probably did this three times on each section of the test. The way I view it, even if you get a 700 on the GMAT, that still means you're missing a lot of questions. So by guessing on questions that you judge you have a low probability of solving, you at least get the questions wrong "on your own terms".