Our standard advice is:
Note: To have your domain resolve correctly, please use ns1.everydns.net, ns2.everydns.net, ns3.everydns.net, and ns4.everydns.net as your domain's nameservers in your registrar's whois database. Without that, none of your records will resolve properly.
We say that because, by default, each Primary Domain we publish will list exactly 4 NS records, for the servers we advised you to list in WHOIS. Now, our standard advice is somewhat simplistic. If you choose not to follow it then you could easily arrange for the Wrong Thing to happen and for silent DNS failures to plague you and your site's visitors, but if you really do know what you're doing, and you use a DNS config checking service, then the Right Thing will likely happen and life will be good.
The last sentence of section 4.2.2 of the spec is important when you are choosing your set of nameservers. It explains that for the Domain Name System to function correctly, all nameservers must be in sync with each other. So the parent nameservers (run by the same registrar that publishes WHOIS data) must all agree on a set of nameservers for your domain, and each of those nameservers must be in agreement with the registrar nameservers. It can be a little complicated, which is why you should verify that DnsReport does not flag your domain with any red errors.
If you have a Primary Domain on EveryDNS, then we will publish NS records for at least the 4 nameservers mentioned above. It happens that ns{2,3,4}.everydns.net are always guaranteed to give the same answers that ns1.everydns.net gives, so if your registrar does not list all of them in the NS-set, but we do, it is not the end of the world. Such a setup would not comply with section 4.2.2, but it would not yield the inconsistent answers which the spec tries so hard to avoid. You may see that DnsReport labels some of ns[234] "stealth nameservers". If your registrar is able to list them in WHOIS, then you should. If your registrar can only accommodate two or three nameservers, then so be it; the DNS will still yield good answers.
If you have a Primary Domain on EveryDNS and you are also listing non-EveryDNS nameservers, then you are raising your hand saying "I am an advanced user", and it is absolutely imperitive that you ensure DnsReport gives your domain a clean bill of health. The use of Dark Magic without careful forethought can lead to pain for you and for anyone visiting your site, though it may be hard for the uninitiated to divine the source of their suffering and of their timeouts. Think twice before ignoring red DnsReort errors. Your registrar must be able to list more than 4 nameservers. You must use "Add Record" on EveryDNS to list NS records for your other nameservers. You must arrange for each of those nameservers to make automated AXFR zone transfers from EveryDNS. Any mismatch in NS-set or in SOA zone serial spells serious trouble which should either be repaired at once or reported to <support@EveryDNS.net> so we can assist.
If you have a Secondary Domain on EveryDNS and you are also listing non-EveryDNS nameservers, then you are raising your hand saying "I am an advanced user", and it is absolutely imperitive that you ensure DnsReport gives your domain a clean bill of health. In this case, you have the flexibility to list fewer than 4 of ns{1,2,3,4}.everydns.net. Please read FAQ item 1 and then list one or more of our nameservers in your NS-set. The spec requires that you use the identical set of NS records in your WHOIS data and in your zonefile, and it also requires that each of your nameservers stays in sync with your master nameserver. Again, DnsReport is your friend!