My experience with the map matches Simon Appleton's. When I played as the Gauls, factions generally didn't attack me until they got my map information. Then, they'd make a beeline for my least-defended cities. I pretty much stopped selling map info and these problems ceased. So now I sell it only to folks far away and unlikely to be a threat. So, playing as the Gauls, sell to Armenia? Parthia? Sure. Germania? Hm, not so much.