Explain to your students the way the scoring system works (+1 for a correct response, -¼ for an incorrect one). Explain how random guessing, statistically, is a break even. Explain how, if a student can eliminate an answer, the odds say she should guess. But leave it at that. Because if you don't, and she guesses, and it costs her, you'll be Trent from Swingers. You'll be maligned for giving good advice, because you insisted on it too strongly instead of letting your student make the final call.
There are some things you, as a tutor, should insist on. Writing out algebra instead of doing head math, for example, costs the student nothing although he may resist. This is a good fight, because when you win you'll probably make an improvement in his score. You're changing his habits, and causing him to do something that will at worst, make no difference, and at best, drastically reduce his careless errors.
When you have the guessing fight, you'll often find that even if you win, you're not making a huge score difference. That's because guessing has a lot to do with luck. SAT guessing strategy is just a way to make it slightly more likely that a student will get lucky. Once in a while, your student might actually get unlucky and lose points. And then it won't matter that you're right. When you find yourself having to defend your guessing strategy to a student who is looking at a 690 instead of a 700 because of guessing, you're in a bad fight.
I like to run this experiment with students on practice tests. And then, after we've done a few tests that way, I shut up about guessing and let them make their own decisions.
I always double down on 11. But I don't make my friends do the same when the stakes are high.