I think it has to be quite bad for you to get special consideration. If it's something quite trivial like a headache, a cold or a stomach ache, I wouldn't bother, but if it's something a bit more serious that you could see a doctor for, it's worth getting them to write you a note. After your exam, give it to your exams officer and ask him/her to send it off to the exam board for you. I'm not sure whether you get marks added on or whether they just mark it more leniently than they would otherwise, but in any case, every little helps!