In my experience, it only connects to carrier Wifi if one of your known networks isn't nearby. So while at home, you will always be connected to your own Wifi so everything local (printers, TVs, etc.) will continue to work.

The problem especially is when you are driving and stop at a red light, and it connects to carrier Wifi which will no longer work in 30 seconds, causing streaming music or anything else on your phone to have a momentary loss of connection.