I had a white iphone 4, 4S, and now iPhone 5. I never had issues with the white becoming dingy or dirty. Nothing around the home button etc.
I don't know what other people who have had those issues are doing to their phone but it can't be normal. I mean, they must touch it with nasty, greasy, unwashed hands or something.
I don't baby my phones at all. Never broke one and they all look pristine without using a case. I have either sold or traded in each one upon the next models release, and receive maximum value.
Buy with confidence and enjoy the beauty of the white iPhone.