So you're all set to sweat out the perfect opening?
The dictum of the King of Hearts has its place --
The White Rabbit put on his spectacles. `Where shall I begin, please your Majesty?' he asked.
`Begin at the beginning,' the King said gravely, `and go on till you come to the end: then stop.'
-- but it is about structure, not about the writing sequence. For one thing, even if you outlined your story, you may not know where it begins. (Doubly so if you didn't.) It may turn out you need your story considerably more medias in the res, and everything you put in this scene has to be hacked out and slithered in elsewhere as backstory. Or else the story is unintelligible without enough backfill to turn your opening chapter into a welter of past-perfect, and so you must move backward in time. (You may even have to use some bridging conflict to keep your character going before the real story kicks in.) In that case, it is more likely that you can salvage the scene, but it is very likely to need extensive rewriting; opening scenes do not read like scenes in the middle of a story; for one thing, a lot of exposition you deftly larded in is redundant and will stick out blatantly, and so must go.
It is a byword among musicals that the last number to be written is the opening one. It sets the stage, and so you need to know what stage it is setting. In writing, too, the hook is a matter of revision more than writing.
And plopping down any old opening sentence does have the advantage of getting you off and going on the story. When I was very new to writing, I would tell myself that I would come back to write the first sentence later and start with the second sentence. Indeed, "Once upon a time there was" gets the story moving with admirable briskness, because you must immediately state what there was. Then you revise it out later.