I've got a NW6.5sp3 network of 6 servers in our main office. One
server is configured as a Single time server getting its updates from
an outside time server. The other 5 servers are set as secondary
servers referencing to the single server.

I also have 5 remote sites with NW5.1sp8 set up as site-to-site VPNs.

Is it ok to configure each of them as secondary time servers pointing
to the single at my main office?

If not, what is the best way to do this? Thank you.