This is one of the items that is tested in your Windows 8 App when you use the WACK.
If you are in Southern California, please email this post for more information on getting a token for your app by June 30, 2012!
Rely on local, packaged content as much as possible
Local assets (images, data files, etc.) load quicker than assets you download from a service or website via a network call. Common caching techniques should apply to your Metro style app.
To minimize unnecessary or repeated downloads, include all the assets that your project needs within your application’s package.
If you must use online assets cache them locally so you can reuse them. Apply a policy that meets the right balance between proper expiration times, managing local state and how much space you use in the local drive.