There's a certain strategy to remarks.
First, consider the subject. Essay subjects are generally quite liquid in marks while maths and sciences aren't. Even if you put down many good points in an essay, many other factors ranging from handwriting to the examiner having a bad day can bring you down. Maths, of course, isn't affected nearly as much and it could well be the case that your grade contains lots of benefit-of-doubts. If you're more than about 2UMS off in Maths, I would not advise getting a remark at all, but rather a retake. Maths modules tend to build on each other so you are likely to be in a better position when retaking, but it's up to you. The point I'm trying to make is that you shouldn't really be trying to scrape an A at AS.
Second, consider how the UMS has added up to give you the grade you have; is it one module dragging the rest down, or are all of them averaging out? In the former scenario, the decision is easy. If you feel the module grade is (very) misrepresentitive of how you feel your exam went, you should go for a remark. If you feel the mark is unfair but have doubts about how well you've done, get a photocopy first and go over it with your teacher- better to spend £10 rather than lose £40 and some marks. If all modules are a bit iffy, seek advice from your teacher.
From experience, here's some speculation of your 'chances', although a photocopy seriously wouldn't go amiss:
'I'M OUTRAGED WTF ARE YOU DOING [AQA/Edexcel/OCR]' = 70%+. e.g. My Computing coursework exam C to A.
'That's a bit low, isn't it?' = 50%, but probably by only a few UMS marks.