What you have to remember is that even with hairstyles that cover the hairline,from time to time whether through wind,swimming or running your fingers through your hair etc... your hairline does get exposed.
If you had a hairline that looked off or strange then people could notice something off,so why take that chance really?
After all we are all trying to achieve the ultimate in undetectability.
A natural hairline does give you that extra confidence,especially in circumstances where it could be exposed, so i would always recommend a good natural hairline, whether your style dictates covers the hairline or not.