I have found that this helps a ton on PMUA. I attribute many of the upgrades I got as a 2P to doing this. Be warned, though, that if the upgrade clears after check in but before you arrive at the airport, you'll be unable to actually assign yourself a first class seat until getting to the airport and having an agent remove the economy class segment from your reservation and check you into the first class segment. You will get an error if you try to return to online check in or try to use a machine at the airport.