I always try to have all devices in/on my network to use my core in-house DNS servers.
My DHCP servers on my customer NATted networks/devices to customer machines DHCP assigns my DNS servers.
I never use the NAT device as a DNS server or forwarder , it just slows down DNS requests.
I never use outside public DNS servers (example 184.108.40.206 or 220.127.116.11 ), it just slows down DNS requests.
By having everybody use only two or three of your core DNS servers, you DNS servers get really fast on answering DNS requests because they build up DNS cache
By having everybody use only two or three of your core DNS servers, they respond much much faster than outside Internet located DNS servers.
It really bugs me to see slow response times to locate and load web pages. With thousands of machines already using my core DNS servers, there is a really high probability that any DNS request I make will probably be found in DNS cache ( instead of having my DNS servers go look it up ).
North Idaho Tom Jones