Depends on the subject. Sciences if you don't miss too many questions you can still get a good grade. Stuff like geography is much harder however because missing a question say in unit 2 will lose you around 1/7 of the marks and it is hard to get very high marks in a question (missing a single question can make you drop 3 grades).
Advice I would give is look at time available and the number of available marks. Compare these to then see how much time you should spend per mark (e.g. 50 marks in a 100 minute paper, only spend up to two minutes per mark, so 6 minutes on a 3 mark question).