Two times in a row I've been entered for a bunch of resits (ones I didn't ask to do).
In Jan I was entered for both AS Chemistry Units when I only wanted to do (A in one, wanted to resit the B). When I realised the mistake, it was too late to "withdraw" or whatever, so they just said don't turn up if you don't want to do it. I thought I'd turn up and see how I did, actually ended up going up a few UMS (except in ione exam I went down, but hey-ho - I had the previous results to sit on which weren't terrible). Similar story for pretty much all my subjects.
I'm having a gap year come September 2012, and I'll be applying then, so I don't think resits should have a massive impact. After all, it's all done within the 2 year period.
If I sit all the exams I've been entered in for Summer 2012 it looks like I've virtually done an AS and an A2 in one exam sitting (for Maths and Chem). :P
If you didn't have to pay for it, don't cancel it, just don't turn up if you don't feel like it'll benefit you.
The only time I'd watch out for resits is if you don't hold a Cambridge offer and are a potential applicant. You'd want to come out with 90%+ in all exams (without resits) on your first try. (for the supplementary form (SAQ?)) (once you have an offer you can retake all you like to make sure you get your offer - or at least that's what I've seen people do :P) Other than that, I don't think too many universities are going to ask too many questions providing it's all done within the 2 year period of A-Levels.