This is how most people have their browsing traffic nat'd. Because the firewall keeps all of the traffic together in a session, STUN traffic should not be an issue.
This is also how I've configured my firewalls, and none have issues with Hangouts or other messaging apps because of it.
I thought the same thing.
But when we test it, we never receive the bindResponse. I don't know why.
My rules is
LAN to EXT with STUN application to any port to the URLs stun.l.google.com, stun1.l.google.com, stun2.l.google.com, stun3.l.google.com, stun4.l.google.com
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