Not the guidelines you must follow in order to pass the Marketplace ingestion process – these are the guidelines you should keep in mind and follow in order to make your app fit in well with the overall Windows Phone 7’s Metro UI.
The look and feel of the app is critical to user’s enjoyment of using it. It has to work correctly, but it also needs to work in a manner that the user expects. Surprises are not always delightful and can mean the difference between a good and bad review from a user. Following the Metro theme reduces those surprises because the user knows how your app works because it works how the phone works.