Monitoring Cisco ACI - Polling engine in or out of the ACI fabric?

I think it would primarily boil down to the specifics of how that network will be built.   If the in and out segments can both talk to each other then just put the poller closer to where the majority of the devices live (probably in fabric if this is a network centric instance).  If the networks can't easily route then you could also do multiple nics on the Orion server so the traffic just goes out the appropriate interface depending on where the endpoints can be accessed.