You have to state all grades you have achieved on UCAS. If you don't, it is considered fraudulent and that means instant rejection from all your choices. Therefore, it's important you do mention that grade, even if you're not overly happy with it.
Edit- Oh sorry, just realised this wasn't what you were asking about In that case, it's better to wait till AS if you think it will weaken your profile. Besides, an extra AS grade won't do you that much good anyway tbh.