Most machines store their internal clock in Universal time (UTC) a.k.a GMT. The local time is then configured via a timezone setting, commonly with an environmental variable named TZ. In other words, it should not matter which ntp source your machines use, if their timezone information is correct they will all keep the correct time.

If you have 600 machines to synchronize, you may wish to setup one master time server which is under your control, have them sync to that one and then have it in turn sync to a stratum-1 server. (i.e. distribute the load)