Depends on the situation, IMO.
If you are doing a ZC, already have 20% of the zone, anyone that come in is a threat, including the bluesuit that is running to the exit. You won't be able to have the zone if there is any contest. People should be aware of this when they come in.
If you are just wandering in the wasteland/core and meet someone, it is up to you and that someone to decide the next step.