Your data costs are already built in with the BIS contract, so wi-fi isn't really necessary.
I have my wi-fi on all the time, makes no noticeable difference to battery power at all. As soon as I walk into myu house it picks up the Cell-C router. Yes, BIS is included, but it is just so much faster on wi-fi that I am quite happy to scrifice a little of my cell-c data.