You could certainly leave both mobile data & WiFi turned on all the time - as long as you have a good WiFi connection your phone will use that; if you get out of range of the WiFi connection then your phone will automatically switch to using mobile data.
However, to save the maximum amount of your data plan, I'd recommend leaving mobile data turned OFF, and only turn it on when you need something then turn it off again.
Sent from my rooted, debloated, deodexed Sinclair ZX-80 running CM -0.001 using Tapatalk 2