If you can definitively eliminate two or three answer choices and guess without prejudice, then it probably makes sense to guess. Otherwise, don't.
The SAT penalizes each incorrect response with a onequarterpoint penalty to discourage random guessing. So, should you guess? And if so, when?
On the Math test, there are 54 questions. Let's say you guess on 10 of them. What effect would that have on your raw score?
Number of Answer Choices Eliminated  Statistically, the Number You’ll Get Correct  Statistically, the Number You’ll Get Incorrect  Effect on Your Raw Score 

0 (oneinfive chance of getting it right)  2

8

+0 
1 (oneinfour chance of getting it right)  2.5

7.5

+0.63 
2 (oneinthree chance of getting it right)  3.3

6.7

+1.67 
3 (oneintwo chance of getting it right)  5

5

+3.75 
Statistically, the effect on your raw score is nothing if it's a pure guess (i.e. no answer choices eliminated) and negligible if you're able to eliminate one answer choice.
Mind you, this is what is supposed to happen statistically; however, the SAT folks do a great job making wrong answers look right, resulting in students being notoriously poor guessers.