i use my phone to fly and dont shut anything off, havent had a problem in 9 months of flying although if someone calls or txts or some other notification pops up and blocks part of the screen it can be slightly annoying, but its not going to cause you to lose control or anything. Ive taken calls and held the remote up to my head and just let the inspire hover before, lol! Its personal preference really.
I fly with my iPad WiFi switched on and tethered to my phone. My iPad is WiFi only, no cellular. I have had no issues with signal so far. Take off in ATTI mode, make sure your flight controls are good before switching to GPS.
The manual says one thing. But the app supports streaming to YouTube so make of that what you will. Either way, fly with additional signal in the air at your own risk.