My belief, based on experience, is that the U bucket is only guaranteed to equal the F bucket once OLCI starts (perhaps a little before). So it is possible that an F seat will open.

In spite of assurances from Alaska to the contrary (which is hearsay, since I'm not invited to Gold lunches as a lowly MVP), I've seen plenty of evidence in recent flights that the F class waitlist does not transfer to the airport. So, if you are an Gold/MVP and you aren't upgraded by the time OLCI starts, I would check several times in that 24 hour period in the reservation screen to see if they are selling an F seat. If they are, complete your OLCI and the link should appear to upgrade you.

Once OLCI starts, it's pretty much a free-for-all. MVPs, Golds, and paid upgrades (H class and higher) can all upgrade at anytime during that window, with no preference for status.

If F seats still exist 40 minutes before flight, the gate agent will call by name each Gold or MVP to exchange your boarding pass, so make sure you are there. Once (s)he does that, it will be opened for paid upgrades to any customer on any fare. It doesn't hurt to confirm you are still on the waitlist when you arrive at the gate.

If F space is open 10 minutes before door closure, the gate agent will come onto the plane and offer you an upgrade. This is a nice perk, as several airlines (including Delta) do not offer this service.

One thing not mentioned so far is that you must manually request an upgrade on each reservation. Remember to do this right after your purchase, because the time stamp on that request will be the tiebreaker if two people of equal status and fare are fighting for the same F seat.

NW/DL upgrades are still in a state of flux. The best thing to do is put in your MP number into those reservations (you can now do this online now) and hope for the best, or make a polite inquiry at the gate.

Sorry for the long post, but it seemed like the OP wanted some more detailed information. I should write a guide on this.