I'd do them if:
a) You enjoy them
b) You're okay at maths
c) You didn't get the highest possible grades at GCSE because you either didn't work or other circumstances affected your performance. If you put in all the work, and tried really hard, and still didn't do that well - then I don't see the point, really. It only gets harder from here on in.
If you're just doing them so you can apply for medicine or dentistry, I'd maybe rethink it. It's hugely competitive to get in and you'll be up against a lot of people who did really well at GCSE.