brainmelt I don't think it makes much difference either way, but if looking for a slight advantage I'd start at the front. The first questions are easier, they are there to calm the student down and get them warmed up. Also, to be honest, an easy mark counts for exactly the same % as a difficult mark, so its best to ensure you get the low hanging fruit first, I'd have thought.
Saying that, I do think its a really good idea to look through the whole paper briefly before you start. That way you can make a mental note of which questions you want to make sure you leave time to do. you don't want to spend ages staring at one question, and then run out of time for one that you could have got full marks on.