When possible, follow the internal Microsoft Coding Standard* as a guideline for coding. "Guideline" means that it is a way to set a routine and standardize things when it makes sense to do so. Its not a law or something that is iron clad. Not following it will not get patches rejected, though they may be modified when applied. Common sense and sound logic trumps all.
The reasoning why the guidelines should be followed is that anyone can open any file and know where the code is located, its consistent, easy to read and follow.
* minus putting the using statements inside the namespace.