I often tell my students that if they think they won't finish anyway, to start with the longest problems first and only work on them for HALF the time of the test. Then go back to the beginning and work through then. This doesn't mean necessarily that they start with the hardest or the ones they don't know. But they should start with the multi-step, complex problems. That way you finish the longest ones and aren't worried about how much time is left, then jump back to the short and simple and finish it quickly.
Different test strategies work for different kids. If it's multiple choice and essay (regardless of topic) and you can pick you order, I always choose multiple choice first. Then I refresh my memory on key terms by using the multiple choice as a guide for my essay.
