You shouldn't be actively using, interacting with, and providing manual input to your device (be it a phone, garmin edge, or anything else) while cycling. An occasional glance at a navigation app (on a screen that is held by something other than your hand - a clip, stand, etc.) is probably fine - but taking your hand off a handlebar long enough to hold a device, and taking your concentration off your path long enough to interact with it, is asking for trouble.