Yes I do trade in differnt exchanging sites...first because some altcoins is doesn't exist in other exchaging sites. Second, because sometimes there are different rates of altcoins in every exchages. Furthermore, I used also the sites to store few of my altcoins because I believe that don't put all of your eggs in one basket.
That's a good strategy that i'm using quite a lot before but not that often now. The high fees from transferring from ine exchange to another is really a setback for people using this strategy. I mean the price difference isn't usually that far between the exchanges and back when the fees weren't that high, i think its ok. But now, i do it very selectively as sometimes it's not worth it.