edit: oh oops, I thought this post was about physics :/
I think it may sort of apply to chemistry (though the multiple choice in chemistry might be harder than the multiple choice in physics, in which case it wouldn't be too relevant).
I flick through the entire exam for the first two minutes and decide which section has the lengthiest questions. I then spend 2 to 3 minutes on each section, figuring out how long each question will take. The rest of reading time is spent just re-reading over the questions that I think will take longer (usually the ones that give a diagram and a scenario kind of thing).
This usually ends up with me ignoring the multiple choice, since that usually tends to have questions that can be done easily during writing time through elimination etc.
When writing time starts, I start with the lengthy questions first.