I turn wi-fi on at home and off all other times.
I never turn 3G off.
It's a matter of forgetting: it's going to happen from time to time. Now, if I forget to turn wi-fi the correct way, well...eh. No big deal.
But if I forget and leave 3G off at the wrong time I end up with EDGE. Ugh. Not worth it, so I leave 3G on all the time.