On my return flight this last week, I was sitting in 5C on a fairly empty flight. A family of four boarded - the father and one child were seated next to me, while the mother and another were seated a few rows back. Since there was an empty seat across the aisle from me, I offered to move so they could all sit together (this prompted angry glares from the gentleman in the middle seat across the aisle who apparently wanted the aisle seat instead, with an empty middle seat - whatever). During this process, I asked the flight attendant if I could move up to 4d (there were at least 5 empty stretch seats) and was informed that I couldn't. I told her I was a Summit member and she informed me that she wasn't supposed to move up anybody into those seats, "even Summit" members. I was a little taken about, but she did eventually let me move up.

Only once has Frontier called me up during pre-boarding to ask if I want stretch seating (I almost always fly in 11D exit row). I find if I don't specifically book it, it doesn't get offered, even if they're empty.