Guessing, if you have no idea which way to guess is more likely, will not have quite the same result as leaving questions blank. Leaving questions blank will add 0 to your score, while guessing will add a mostly-Gaussian random variable with a mean of 0. The math of this is kind of fun:

http://en.wikipedia.org/wiki/Random_walk

And of course, the central limit theorem is colossally important:

http://en.wikipedia.org/wiki/Central_limit_theorem