The problem with iPhones (and iPads) is that if you put them into flight mode, this also disables the internal GPS
I always thought that it was a bit of a design fault. There are lots of situations where you might want to be 'dead to the world' as it were but still need the GPS. Anyhow, I leave the whole shooting match on with my phone and it doesn't seem to bother anything. You get live updates, weather and what have you while you are flying as well if you leave it on fulll fat mode.
It's quite an amazing bit of kit really cf what pilots had twenty years ago, and it just goes in your trouser pocket when you aren't flying.
Disclaimer: for the old and bold here I still fly with a marked up map and plog.