If you are on Wi-Fi then it doesn't get used anyway so there isn't much need to disable it and then enable it again later when you are out and about and need it then.
I'm on AT&T Unlimited Data plan so I don't have to worry about this but when I was on the 15GB plan I always left my data on and it didn't use nothing.
Leave mine on 24/7 but have the use cell over wifi option off. Got the double data through AT&T promotion so I have 30GB's on my family plan, we rarely hit 15 so have plenty of headroom so I don't worry about it.