If your worried about using cell data while on WiFi, you shouldn't worry about much except during the over night period. Cable, DSL, Internet providers conduct maintenance over night. That's when you may use cell data. Here's a tip, instead of turning off the cell data completely. Simply go to Settings > General > Cellular and turn off the programs that use cell data. Next go to Settings > Privacy > Location Services > System Services and turn off all of those functions. That will stop most potential use of Cell data. I hope this helps.