Getting good GCSE grades definately do matter!
Not only do they allow you to do the courses you want, but if you change your mind about what you want to study on the first week of AS levels then you have a wider option of courses you can change to. Not only that, understanding your GCSE syllabus really well helps with the jump to A-levels (depending on what courses you do) and if you decide to apply to university, a lot of them do take your GCSE results into consideration. I completely understand the cba attitude -we all get it at some point, but revising early disciplines you well because out of the examination years, GCSEs are definately the easiest.