This is something I didn't know and which is probably quite important.
MS does mention that service packs and hotfixes should be reapplied
Important The resource database (mssqlsystemresource) is restored from SQL Server 2005 distribution media. When you rebuild the resource database from distribution media, all Service Packs and QFE updates are lost, and therefore must be reapplied. Before you proceed, see [REBUILDDATABASE] for more information.
However under the [REBUILDDATABASE] entry, there's no further mention of this, so I suspect a lot of DBAs who have to rebuild the master database don't get back to where they should be!
Of course, it does say that the most recent backup of the master database should be restored as soon as possible would would have the effect of updating it, but there is still great scope for error here.
Derek