Hi MTGlenn,
There dont have the official doc indicate it is better install each role per server, personal point is it depend our infrastructure topology, such we can install IIS and AD
role on the same box and it usually work properly, but it is a worse design for the security.
Another site, the migrate support migrations of specified roles and features from one server to another (not in-place upgrades), naturally one role per server will simper to
consider the effect of others role on same server.
The migration guides support migrations of specified roles and features from one server to another (not in-place upgrades).
More information:
Migrating Roles and Features in Windows Server
https://technet.microsoft.com/en-us/windowsserver/jj554790.aspx
Im glad to be of help to you!