A few years ago, checking your phone while at dinner with someone would've been considered pretty rude. But not so much any more.
There's really a formula to it, the more people in the group, the less rude it is to check your phone. It drops quickly to "normal behavior" once the group has 3 or 4 members.
Now, checking your watch (Apple Watch) sends the message you have somewhere else to be. But I bet that changes too, as more and more people have watches and devices that show their messages and emails. Glancing at the screen, then getting out the iPhone only if it's something more important than the people you're with.