I don't know that it matters, as long as it is always available. We actually have 3 LM (virtual) servers that pretty much only have the LM software on it (i.e. each is dedicated). That way we can reboot those individually, but not be affected by reboots of the Server software. We have 3 because we wanted to 1) isolate license that can be borrowed for offline use (Desktop) and the option file never works to the degree we need for this; 2) one for the general gis users, and 3) for the GIS fulltime staff.
So, we do have them separate from our base software, but it isn't absolutely necessary. Again, it could be run on a lightweight desktop OS, it doesn't have to be a server os, but in our office it tends to be auto-booted less if its a server.
BTW - we also have some concurrent Pro licenses on the LM (i.e. more local that using ArcGIS Online named user). But they haven't been used much, yet.