There is the (slight, although ever increasing due to CO's recent acquisition of UA... ) chance you'd be IDB'd unless you have a must fly priority of some sort (you're a United maintenance member, a pilot/crew being repositioned to fly another plane, etc.).

That said, considering the spectrum of E+ prices on some flights (I'm looking LAX-SYD at the moment and see E+ middles for $135, E+ window/aisle for $159, and exit rows [another change I'm going to like] for $169 while not logged in), this may be a worthwhile gamble.