I wouldnt worry about it, Unis dont tend to look at nat5's unless they are needed for entry. Most unis look for good higher grades rather than nat 5's. The best thing you can do to maximise your chances of getting into the uni course you want to is take the highers you are best at/ what you need for entry. Related subjects are also a good idea, for example, essay subjects for english. If you are doing english at uni i would probably recommend at least one essay subject. If you only have english then people who are doing 2 essay subjects + english will have an advantage over you, not to mention advanced highers but thats a while away.
In short, Unis wont care about your nat5's that much, so study hard and good luck in your Nat 5's!