Even though not all of the Casitas buildings are preferred, Casitas is not one of the request options when you do the online check-in. I guess it's because they try to put most of the convention-goers there. We were in Casitas 5 on our last trip and it was our first time in that section. Our room wasn't ready when we arrived, but the CM was able to tell me the room number started with a 5, so I knew it was in Casitas 5 and was happy to wait for it.
Our first stay at CSR, we were in Cabanas 9B - many many years before Business Class came to be. It was a great location. We were there just a couple of years after 9/11 and the parks were pretty dead in Sept back then so we had no trouble getting the bus at El Centro. Since then, it's definitely become a lot more crowded. For this coming trip, CSR was the only Mod with military-rate rooms available for our particular weekend, so maybe it won't be too crowded.