Best answer by Dennis
Wifi can always stay on.
To send an MMS, dta must be enabled. MMS does not cost money, but it goes through the same data pipeline. That is just how the technology works in Canada (all wireless providers).
You just have to watch out for data leakage from your other apps running in the background. To prevent that, just connect to a strong wifi connection and turn data on. This will prevent data leakage