For new hires - keep applying to your higher ranked choices if you get hired at a lower ranked choice. Is it worth quitting your fourth choice after a year to go to your third choice? Doubtful. But for your first choice? I think most guys would say yes up to a point. Three months? Sure. Six months? Maybe. But the reality is every month you're at choice X, and not your first choice, choice X rises and choice #1 drops.
A friend, military guy, got hired by his first choice(AA). Awaiting his class date. Another airline called. He was going to turn down the interview - "go, you have no idea what might happen while you wait for your AA class date." Yup, he got caught in the recallee bubble at AA and AA didn't offer a class date for a year. The company that he almost turned down has worked out better than anyone could have imagined. For him, within months, choice X > choice #1.