Before using should or must, consider other ways to discuss recommendations or requirements, such as the imperative mood for required actions and a phrase such as we recommend for optional ones. Don't use Microsoft recommends or it is recommended. Be careful to make your tone helpful, not bossy. (For information about the imperative mood, see Verbs.)
Examples
You'll need an internet connection to install the upgrade.
We recommend that you wait for us to notify you when we've confirmed that your PC is ready.
Free technical support is available when you register with Microsoft.
Use should only to describe an action that's recommended but optional. Don't use should to indicate probability. If you can't make a definitive statement, use might or rephrase.
Examples
You should back up your data periodically.
If you don't use automatic updating, you should check for updates yourself at least once every week.
If the anti-malware app is compatible and current, it will be preserved during the upgrade.
Use must only to describe a required action.
Example
To save copies of a document in the same location, you must save each copy with a different file name.