Welcome to FT,
Magicman_007!
or should we call you James??
As
me4yankees said, you can definitely doublecheck at checkin (Select Seats) and also when you get to the gate - Preferreds get automatic free upgrades on US, so exit rows will often open up by then.
Also, if you're interested, getting Silver Preferred status on US is relatively easy (spend $25K on one of their credit cards and 10K of your bonus mi get converted to PQM; then fly only 15K mi) and will give you access to exit rows at the time of booking. The credit cards give you preferred checkin and Zone 2 boarding automatically.