If you restore master, you will need to fix the logins for any orhpaned users., Not typically an issue however, if one of the orphaned users was the owner of one of the databases you're restoring, it will cause an error (easy to fix).

Here's a simple script to correct this:

http://stackoverflow.com/questions/2872267/script-to-dynamically-fix-ophaned-users-after-db-restore