Is it always best to check with the gate agent in these situations about upgrades at the airport, or can you actually trust in the GA?

One of the reasons I ditched NW/DL, and moved to AS was because of the wretched customer service by GA. They would routinely leave F seats open, or bump up non-revs to F that I witnessed on several occasions when I was on the F list. I got so fed up with them, I jumped ship to AS and haven't looked back since.