Personally I like 'domain-registrar' better but it seems as though 'registrar' gets used much more. Either way I don't think we need both tags.
Also up for discussion:
domain(213)
domains(28)
domain-name(55)
domainname(2)
domain-names(1)
Though I can these being a problem since some I'm sure some are referring to windows domains and some to website domains.