You can address them with whatever domains you choose; suffixing them with .localhost is no better than suffixing them with .local, .loc, .qwyjibo, or not suffixing them at all: these are merely personal choices you made to give the domains some look and feel, but they aren't intrinsically privileged by the system.

I've taken to having my VMs support mDNS (specifically avahi on Debian), and I can connect to <hostname>.local without adding an entry to my hosts file or messing with DNS.