They don't need to be parked. It just won't show a pretty page, that's all. Some registrars will even let you have no name servers. I wouldn't point the domain to name servers that don't handle the domain unless you have control over those name servers or to name servers that don't exist because I've gained control of domain names that I didn't own that way.
GoDaddy let me have no name servers until they switched to their new interface. I don't use anyone else so I don't know what other registrars allow this, but I've seen domain names with no name servers when doing a WHOIS, but I don't remember any specific one at the moment.