The only instances in which I would skip a question would be when I had absolutely no fucking clue what to do on a math question, and also when an answer for one of the last few math questions seemed way too obvious and I didn't know why it was so easy (basically meaning I had no fucking clue what to do, except this time there was a trap). Honestly, I wouldn't even consider skipping an option really. Put it out of your mind unless you really, really don't know how to do a math problem. If you think you might want to skip a critical reading or writing question, you're probably not giving yourself enough credit, because the only times you should skip is when you can't even begin to figure out a single choice to eliminate. You either need to get 2 or 3 wrong before it even starts to count against you, and once there, you need to get another like 4 for it to count against you again.