Q:

Should i put all my servers into my domain? 

I am setting up a lab/mock-up network and i'm wondering if is considered bad practice to add all my servers to my Windows Domain.  Of course the DC must be part of the domain, and any server that users log in to, such as VPN must also be part of the domain. 
What about servers that don't necessarily need to be connected to the domain?  Like a DHCP server, external web server, or DNS server?  Should these be part of the domain, or only connected to the internal network? 

A:

Only if you like centralized management and single sign on and group policies and security features such as Kerberos. 
If you don't like those things then don't use an AD domain. 
You might have a case for not putting 'DMZ' servers like public facing web servers on the domain, either because of lack of network connectivity or because of security concerns.