[highlight style=”yellow”]Note: This post is about the old SAT (pre2016). The “new” SAT does not have a penalty for incorrect answers. [/highlight]
In short: almost always.
(Note: This is generalized advice; if it doesn’t sit well with you, read this.)
I’ve encountered a lot of misinformation about the SAT in my travels, but the single subject that generates the most confusion and rampant speculation is The Guessing Rule. So here it is, as plainly as I can put it: If you have read a question and thought about it for more than 5 seconds, you should not leave it blank.
Here’s how it breaks down:
Every incorrect answer in a multiple choice section* costs you 1/4 of a raw score point. Every correct answer, of course, gives you a whole raw score point. A blank has no positive or negative effect on your score. Fractional points are rounded to the nearest whole number when scores are compiled.
Imagine two ne’erdowells, Johnny and Morrissey, are taking a much shorter test with the same scoring scheme. Johnny doesn’t give a damn about the test, and guesses C for every question without even looking at it. Morrissey cares even less than Johnny, and just leaves the whole thing blank, opting instead to stare out the window dolefully.
Question #

Johnny

Morrissey

1

C

[blank]

2

C

[blank]

3

C

[blank]

4

C

[blank]

5

C

[blank]

When you guess completely randomly, like Johnny did, what are your statistical odds of getting a question right? Well, there are five choices, A through E, so that’s a 20% chance (or 1 in 5 odds) that you’ll get any particular question correct. Since our test had five choices on it, Johnny will get one of them right, and the other four wrong.
Question #

Johnny

Morrissey

1

C +1

[blank] +0

2

C 1/4

[blank] +0

3

C 1/4

[blank] +0

4

C 1/4

[blank] +0

5

C 1/4

[blank] +0

Note that when you total up Johnny’s score, it’s the same as Morrissey’s! Both get a grand total of 0 raw score points. Now, it makes sense that they both would earn a goose egg — they both did about the same amount of work. It would be unfair to give Johnny a better score simply for picking up his pencil and bubbling randomly (note: this is what the ACT does).
It might be clear by now why the SAT’s scoring system works the way that it does. It’s not to penalize you for wrong answers; it’s to prevent people from gaining an unfair advantage. Say your proctor calls time at the end of a section that neither you nor the person next to you has finished. You put your pencil down like the obedient student that you are, but your conniving neighbor hurriedly bubbles in random guesses for the last few questions. That person shouldn’t have an advantage over you, and the SAT’s scoring system (on average) ensures that she doesn’t.
So the obvious implication is that random guessing doesn’t pay. Why, then, am I arguing that you should guess whenever you’ve had time to read and consider a question? Simply put, because then you’re not randomly guessing anymore, so you’re tipping the scales slightly in your favor. You will still get more questions wrong than you will right when you’re guessing, but even if you only eliminate one bad choice before doing so, the math says your score will slowly go up.
Let’s look one more time at Johnny, but change his strategy a bit. Let’s say now that he’s still randomly guessing, but before he does so he’s putting in the minimal effort of eliminating one choice he knows is wrong before doing so (so his odds of a correct answer are 1 in 4). Let’s also say the test got a little longer…say it’s 8 questions now. That means, statistically, that he’ll get 2 right, and 6 wrong.
Question #

Johnny

1

C +1

2

C 1/4

3

D 1/4

4

C 1/4

5

C 1/4

6

B +1

7

E 1/4

8

A 1/4

TOTAL POINTS

+2/4 = +0.5

When it comes time to calculate final scores, that half a point will round up, and Johnny has just (amazingly) helped his score.
I want to be clear here: this is not going to net you hundreds of points. This might get you 10 points on a test, or it might get you none. Since it’s purely theoretical, it might even cost you points on a particular test if your luck is worse than average (remember, even though statistically you have a 50/50 shot when you flip a coin, sometimes in real life you can flip heads 10 times in a row).
This guideline is kinda like the rules a Vegas blackjack dealer has to follow. I know you’re probably not in casinos very often if you’re worrying about the SAT, but maybe you’ve been to a charity casino night at your school? At my school that was one of the attractions of the postprom party. But I digress. The object of blackjack is to get as close as possible to 21 without going over. If you’ve got 18, but you’re feeling lucky, you can take another card to try to get closer. It might not be smart, but you can do it. The dealer, on the other hand, cannot. He has to stop if he’s got 17 or higher, even though he can see your hand and therefore might know that 17 is a losing hand for him. Why? Because someone very smart at a casino a long time ago figured out that if he always does that, always, then the house will slowly but surely win money, even though some individual players might walk away from the table with more money than they came with.
So it is with you and guessing on the SAT. If you always guess when you’ve read a question and thought about it for more than 5 seconds, you’ll win more points than you lose, even while you’re getting more questions wrong than you’re getting right. It might take some getting used to, but that should be your new guessing rule.
One last note that should be obvious: guessing is for emergencies only. The better way to improve your score (and the only way to improve it more than a minuscule amount) is to learn some techniques to help you actually get the questions right. Hopefully I can be of service there too.
*There’s no penalty for guessing on a gridin in the student produced response part of the math section. Why do you think that is?