There are two very different schools of thought on guessing on the SAT. Both sides have good arguments, and there really isn't a one-size-fits-all answer, so I talk about it with students on a case-by-case basis. One side advocates guessing if you can rule out at least one wrong answer. This is statistically prudent given the 1/4 point guessing penalty on the SAT. The other side is against guessing, even when you can rule out some answers simply because the nature of the SAT encourages students to jump at plausible, but wrong, answers.
However, I recently read about some new research that makes me think that the anti-guess faction may have it right. This study found that people under stress are more likely to think that their decisions will have a desirable outcome, and that they may have more trouble controlling impulsive decisions because they expect things to turn out positively in the end.
So what does that have to do with guessing and the SAT? Well, it should be obvious that the SAT is a stressful situation. Since students are stressed, they may feel more confident about their guessing ability than they should. So, they may be more likely to guess on questions when it's really in their best interest to skip.
This effect may be even more pronounced in students with ADHD. One symptom of ADHD is impulsivity. In other words, ADHD students are more likely to act or make decisions without thoroughly thinking them through. So in a stressful situation, they may be even MORE likely to guess when they shouldn't.
My own observation has been that students tend to guess much more in their best subjects. One of my tutoring students was quick to skip math questions she didn't feel confident enough to answer. But she answered every reading passage question despite answering many of the harder ones incorrectly. Feeling more confident than you should could lead to poor guessing, too.
In the end, it's up to each student to make a good decision about guessing on test day. Just keep in mind that your guessing skills may be a bit compromised when you're stressed, so proceed with caution!
Note: There is no guessing penalty on the ACT, so you should always guess even if it's a shot in the dark! Add one more reason that the ACT is a better choice for many ADHD students.