Your IOC and a host in an archiver appliance should have the same NTP server configuration. Technically, it doesn't matter which NTP server will be used. But better to use a single one instead of multiple ones.
FWIW:
Unless you're broadcasting time, the NTP protocol is designed to run on a mesh of servers. Its algorithms make sure that bad servers are recognized and isolated (ignored) to increase the accuracy and stability of all servers participating in the mesh.
While having a single server is not the worst configuration (the worst is defining two servers: a recipe for a timely disaster), configuring more servers will increase the stability (five is a good number, more won't hurt.)
Cheers,
~Ralph
