Good question - given the way that the GMAT is scored and administered, you really won't have that choice very often. You must submit an answer to a question before getting to the next question - the GMAT is administered on computer, and assigns questions to you based on your performance on previous questions - so the only time that you'll have that "blank vs. guess" option is on the last question you face on either section.

There, most people are probably going to be better off by guessing. In addition to the 20% or better chance that you get it right, there's also the consideration that the GMAT carries a fairly steep penalty for not completing each section. It's not as 100% cut-and-dry as "always guess", but for most people I'd just carry that mantra. If you're interested in the official GMAC take on it, they have a good blog post on the subject:

https://www.mba.com/mbacommunity/MBA_com ... ssing.aspx

In your case, since it sounds like you're not all that familiar with GMAT administration, I'd highly recommend taking a practice test (there are several free practice tests linked at the right of this page) to get a feel for what taking the test will be like, and to also gauge how you'll want to prioritize your study time.