Before United started charging extra for it, and had a mechanism for it to be charged onboard, I occasionally had a FA move passengers up if E+ was unusually empty. Now that they have the charge on board machines, never ever, and they usually aggressively announce and chase passengers out who haven't paid for it.

That said, flights rarely go out with many empty E+ seats, as UA does oversell E- and op-up at check-in. If you don't already have an assigned seat, you might consider playing upgrade roulette and get assigned one when you check in (usually more successful if you wait to check in at the airport). Of course, you may end up in a middle seat in the last row of E next to the lavs.