I leave my data on all the time and am usually on WiFi. I can't see where I use any of the data while on WiFi. Of course there is a boost setting that will use both at the same time, but I never use it.
I get 4gb data on my phone every month. I am lucky if I use 400 mbs on 4G. It shouldn't take data off. Im On wifi most of the time. Home,my daughters home, bars, public transport etc.