If you want to study a science-based subject that is competitive, or at a top university, then it's probably best you do it. I've never seen anywhere that says it's essential, but if the competition is high then you'll want to get as much of an advantage as possible. It also helps with A-level science. If you have the ability and the interest in science then I think you should do it.
Obviously if you're not looking at this career path then it's not as important. It helps with A-level science but isn't essential by any means, and yes when applying to uni AS grades are more important. It also helps if you have a passion for science, if you are completely uninterested in science then it will probably just seem like a chore.
Probably best you speak with your parents and teachers about this, but it is your decision at the end of the day.
EDIT: A lot of people saying that it really doesn't matter - it depends on what you want to do in life, if it's what I described above, then it DOES matter...