Windows (Admin) Credentials = windows machines (Try to make a service account and deny interactive login)
Linux/Unix = local account (SSH) doesn't have to be SUDO/admin but you get more info that way I believe
Switches/Routers = SNMP Credentials (I suggest Read-Only credentials)
Printers = SNMP (usually default PUBLIC works unless they've specifically been set to use something different)
Phones = SNMP (usually default works)
DVRS, SANS, Cameras, PDU's, Alarm Systems/Door panels = usually SNMP - though may have to enable on devices
VMWare Hosts (Not VCenter, but it scans hosts) = local account made on host (SSH)
Apple - Local SSH account, like Linux/Unix - though SSH has to be enabled on the computers (its usually not)
that's all I can think of off the top of my head
I generally, once I have scanned the switches, look at the ports on them, and if there are just MAC addresses on the ports, versus an asset, I know I've missed some stuff (as well as scanning errors but viewing the switches helps me see it graphically)