Mauve (11/14/2013)
I assign all of my SQL Server logins to the tempdb as the default database. NEVER master. This way no harm can be done by an application that just connects to the server without specifying a database name as part of the connection string. Ideally, I'd like to see an option of "none".
All users except sysadmins get tempdb as a default - anything they add there gets cleaned up next patch/reboot cycle, regardless.
Sysadmins get master, in case tempdb isn't available for some reason (tempdb volume failure, and so on).