From my time majoring in Arch, I'd say the rule of thumb is:

"Is the culture the body came from vanished or changed to the point where no one has a personal stake in it."

So for example, vikings are long since gone. Modern northern europeans are generally a completely different culture, therefore not grave robbing. Same with Ancient Egypt, Ancient Rome, etc...

Indigenous tribes in North America and Australia for example, still very much around and still very much grave robbing (though that opinion is controversial)

Basically, if the existing culture still shows reverence to those ancestors...leave them alone. If the existing culture no longer honours them as ancestors, dig baby dig.