I think it depends on what kind of science you want to do! Getting perfect grades is NEVER a requirement, so don’t ever worry about that, but making sure you understand the basic ideas is, and then build on that with what you are interested in. Getting good grades in the things you are interested in and want to work in is the best thing to do, but make sure you have a solid foundation in the other areas too.
To give you an idea of my background: I did biology and chemistry, but I decided I didn’t like physics as much so didn’t do it for a GCSE grade. I studied up until we had to make our choices then realised I wanted to focus on the others. I made sure I could understand the physics though, in case I would need it in the future, but knew biology and chemistry were going be more important. And it’s always good to make sure you have a firm understanding of maths, as all sciences use maths and statistics to analyse data.