5th and 6th level can be fine. When I hit Orkogre castle, I'm usually 6th level with a Nuclear Blast or 2, a few Words of Death, Asphyxiation, and all the other maxxed spells. So, at an advantage....IF however, you wait til you're 8th level, watch how easy it suddenly becomes to not be affected by the spell effects. Advantages to a higher level are nice, but problematic in that you lose a chance to get the maps, you have to fight "other" things that, at lower levels, are harder to bear, etc.
Personally, I do Old City before I leave New City. Old City is too easy to do when you get the Rat info anyhows, so doing it early makes it a bit more challenging, and gets those levels you need by OrkOgre time. Especially if you're mutli-classing a lot.