Is it a good idea to use NLB to load balance DNS requests to primary and secondary dns servers on port 53?
A modern server can handle 100kq/s or more, it would take a lot of DNS queries to get overloaded. Even most small/medium ISPs just have a primary and a secondary (sometimes a tertiary too) servers.
Many of the root servers are clustered, but that's primarily for redundancy. Root DNS servers commonly see 50kq/s (more on particularly busy days like holidays). But the root servers are a bad example to use for a typical network.