Depends day of week I fly and route. I've gotten Op-up'd into F on this route AFTER being SWU'd into C, on a W ticket. Other times I've flown IAD-GVA on a V and missed it. I try to judge relative fullness of E+ compared to E- to make my decision. Also depends on price differential. I'll go up to $100 more than W each way. Sometimes that ends up being Q or V, sometimes still W