I can't really remember the film (I remember thinking the film wasn't that good - though I know most people like it). But in the book, the twist is less of a twist, it's suggested all the way through. If I remember, it's suggested on the first or second page. You might not pick it up on your first read, especially if you're not reading too carefully, but it becomes super obvious if you re-read it. So, at least in the book, there is no way to not have the twist - it wouldn't make sense without it. Is that the same in the film (I saw it a number of years ago and found it pretty forgettable)?
It's one of those rare beasts, a film based on a very good book that is at least as good as the book. I think the twist is key to the message of the film, but obviously it's a lot harder to disguise and convey effectively on film. I think Fincher did a very good job disguising it in a mass of trippiness and confusion.