I just got back from a nice trip in the area. I OC pretty much everywhere I go and not a comment was made to me. I did catch a few raised eyebrows but that was the worst I observed. In fact I saw several others OC myself including the checkout lines at the brand new Wal-Mart store in Kalispell.
As far as State law goes, as long as the gun is fully exposed, you can go pretty much anywhere you wish except prohibited buildings like the Post Office, Government buildings, schools and the like....Unless the owner or person in charge ask you to leave or disarm.