I saw a post earlier this week from Buck Woody called Don’t Mess with the System Databases and I agree. In general.
You don’t want to make fundamental changes, like changing permissions, without understanding exactly what the implications are. If you are not sure, or can’t find any information that says something is safe, like removing “guest”, you should not assume it is. Test on a non-production server first, and be ready to roll this back after you apply a change.
I saw another issue on the PSS blog as well, which leads me to believe that you ought to not mess with the system databases.
Query them backdoor as needed, especially when trying to roll up information out of msdb, but don’t go changing things you don’t understand.