Infrastructure systems are best assigned a static IP address. DHCP is a crap shoot, you don't really know where a device is going to land, whether or not a lease will renew at the same address, etc., and also comes with the risk if the DHCP server runs out of leases or craps out, your NAS might "disappear" which could be really annoying if you're using it. It's also the only way to configure more than a single network interface, which is very common in all but the simplest networks.