Bad guessing hurts your score; good guessing helps your score. I'm serious. Only two things hurt your score: blanks and errors.
Here, look at it this way. Every time you get to a question, you have two options: answer the question or don't answer it. Every time you don't answer a question, you lose 10 points.
Now, if you answer the question you'll either get it right or get it wrong. If you answer it correctly, you get to keep your points for that question, and you lose nothing. If you answer it incorrectly, you lose on average 12 points.
But notice that when you're answering a question, you don't know beforehand whether you'll answer it correctly or incorrectly. A blank is a guaranteed loss of 10 points. By answering a question you're risking about 2 points to save the 10 that you'd otherwise lose for sure.
In short, guessing sometimes helps your score, but not guessing always hurts your score. You can learn more about the art of good guessing (and the logic behind it) in The SAT: Types of Questions and Answers and The SAT: When You Need to Guess, Beware Your Hunches.