Leave Data On or Off?

So it seems that I need to have Cellular Data ON in order to send/receive MMS. However, because I don't want to go over my 2GB plan, I tend to keep it off unless I need to use data.

I also keep data/wifi off to preserve battery life.

I'm not sure what to do, or what everyone else does. If I leave data on, wouldn't Facebook updates, emails, etc start using data?