Straight answers
Should I guess on the SAT if I don't know the answer?
Yes — always answer every question. The digital SAT does not take points away for a wrong answer, so leaving a question blank can only cost you and a guess can only help. The smart version of guessing is to eliminate what you can first, then pick, then move on.
Older versions of the test did subtract a fraction of a point for wrong answers, and that rule is the reason many parents and older siblings still say "don't guess." It no longer applies to the digital SAT. Wrong answers are not subtracted; your score reflects the questions you answer correctly (and their difficulty), so there is no downside to filling in something.
A guess works best when it is an educated one. Most multiple-choice questions have at least one option you can rule out quickly — it contradicts the passage, it answers a different question, or the math cannot possibly produce it. Removing even one option moves a blind one-in-four guess to one-in-three, and removing two moves it to one-in-two.
Time is the real reason to guess on purpose. Each module is timed, and once you submit a module you cannot go back to it. If a question is eating your time, pick your best remaining option, flag it with Mark for Review, and keep going. If time is left at the end of the module you can come back; if not, you already have an answer in.
Student-produced math questions work differently, because there are no choices to eliminate. You can still enter a best attempt — a blank is a guaranteed zero — but these are better handled by working the problem than by guessing, which is one more reason to practice typing answers in the exact format the test accepts.
Practice the habit before test day: in Brainrise drills, answer every question and then look at your reasoning on the ones you guessed. A guess that turned out right is still a gap worth closing.