I hear there's no longer a guessing penalty on the new SAT. If this is the case, should I always guess on all the questions I can't figure out? Is there any situation where I should avoid guessing?
There is, as you say, no guessing penalty on the new SAT. This means that answering a question incorrectly and not answering it at all will both have the same impact on your score (no points added or subtracted). You should fill in an answer bubble for every multiple-choice question on the test. You have nothing to lose!
That being said, you shouldn't take this as an opportunity to put less effort into figuring out the correct answers to tough questions. You should guess only when you're truly stumped or when you know you don't have enough time left to answer the question. I would also advise against blind guessing on the Math grid-in questions just because it's not worth your time to fill in a random number.