The Math section is ordered from easiest to hardest questions. The last few will always be very, very difficult. So there's no reason to race to the end to get to hard questions which you have less of a chance of getting right. It's better to take it even SLOWER on the easy to medium questions so that you're sure you're not making any silly errors and giving away precious points. Remember, all the questions are worth the same amount. Making two silly errors while racing to the end to answer one hard question correctly is a DISASTER.
The same goes for the Grammar section. And while only the sentence completion questions in the Reading section go in order from easies to hardest, it still makes sense to use the same strategy for them.