Tiger Shark has a very good point about it being his network to run scans against at will. For you up and coming network admins keep that in mind -- know what you're doing as best you can (so you can restore from backup in case you fudge something), but don't hesitate to do everything you can to KNOW your network, no matter what that requires. Scanning overcomes ignorance.

And even if you are not the admin in charge, networks are run with policies, administrative overhead, and supervisory authority. Find out who's in charge and what the policies are, and then feel free to scan away if it's permitted. Some admins will be more open to this than others, but then you have at least gained that kind of knowledge.

Isn't this one of the primary mantras of this site: knowledge is power?