Have just booked and I am in seat 22D. I'll live. Or I'll do online check-in. Considering that I just did PHL-SCL-ANC-SCL-PHL (four flights of 4+ hours each) in seat 30D on DL, this should be a breeze. I'm doing the outbound on UA in F.
And yes, I did sign up for EarlyRewards, although I don't expect to ever be able use those miles for anything! DEN is not a typical destination for me.