Correct. With clients logged off, then there is no way that any program including AC can be connected to the database. So, any backups on the server won't have problems. You will still be able to run antivirus programs if necessary.
I break this rule all the time as I tend to forget to log off my exam room computers.
Of course, a lot of this is easier now with VSS and backup open files with some of the better backup programs.
