Do you actually have devices that insist on using their hard-coded DNS server IPs and refuse to use the DHCP-provided one(s)?
From limited personal experience, I've had a couple devices that always tried to use "their own" DNS server, but when blocked, always fell back to using the DNS server from DHCP. (and frankly, I wouldn't want a device in my network that refuses to use my DNS servers)
I imagine you could make this transition/research fairly painless by logging the outgoing DNS traffic and then gradually moving these devices to a blocking rule, and finding out how they react.
[ corrections always welcome ]