I just poked around Ticketmaster's site, and Houston is by far the slowest to sellout, as best as I could tell. Other cities that have yet to sell out (Tampa, Pittsburgh, along with second nights in Pasadena and Chicago, are easily 80% soldout, so far as I could tell; for some reason, you couldn't zoom in for night 2 in E Rutherford).
Then again, it wouldn't show how many GA tix were still available for Pittsurge, so my numbers could be way off when I say that Houston is the slowest sell.
Also, it looks like they've added a show in Louisville, at Papa John's Cardinal Stadium. That'll draw people from Cincinatti, as well as places as far south as Nashville, with another 55,000-ish tickets to sell.