you don't have to declare your AS results. my school wouldn't actually let us declare our AS results (if we wanted them included, it had to be in the personal statement or reference) so it probably won't make a blind bit of difference to the universities, who won't know whether or not you're resitting. the only problem, as i think someone has already pointed out, is revision - whether or not you will have the time to work for a resit. that depends entirely on your subjects and the type of grades you get. if you do history, for example, and all three of your modules are a bit dodgy, but not terrible, then you'd have to redo all three, which means relearning a lot of material for an AS resit whilst studying an entirely different period of history for A2. for something like a language, however, no extra revision is required so doing a resit is fine. to be honest, though, i wouldn't stress about it. if your results are bad then you can resit and the universities will be none the wiser.