So, Phoenix actually replaces your channel state with the channel state that's on their servers every time you start the app. But they do still encourage you to open the app from time to time. They even give you a notification if don't open it for too long.

And yes, because you aren't routing, you're only receiving and sending, you're good with not having it on with 24/7 uptime, but having your channel state replaced does mean you're just straight up trusting Phoenix not to close the channel with an old channel state.