Not entirely true. Some contacts in Nova Praetoria will stop talking to you at level 8, Imperial City at level 15, and Neutropolis at level 19. It's funky that way. So you're much better off turning XP off at levels 7, 13/14, and 18 just to be on the safe side. 13/14 is a little funky in that once you ding level 14, you may start to get offered Neutropolis story arcs, so there's risk that if you ding level 14 and aren't careful, you may start going down the Neutropolis path before you finish Imperial City, but you'll still get missions from Imperial City contacts. Just need to be really careful.