Well, it's mostly preference. If you were going to have multiple namespaces in the same file, and wanted different things exposed to each, you would put the using directives inside the namespace (important distinction: the ones at the beginning are called "directives", the ones in the code that define a scope for IDisposable objects are called "statements"). Otherwise, outside.
VS templates put them outside the namespace, so if there's an accepted style, that's it. Of course, if your instructor or company has a different style guide, follow it, but I like to stick with MS's recommended guidelines when I can.
I like to stick to the 'by the book (if the book is written by MS)' styles when I can, it's just that when I use nArrange a external tool for VS that arranges the code, puts it inside so I wondered 'Why is it doing that?', and thus this question came to be.