I always used to be in the you-can't-see-it-til-its-finished camp. Completely. No ifs or buts. But having gotten used to the idea of prompts and brain-storming with other writers, I now find it much easier if I have someone to bounce ideas with. And that means they have to know the ending (or possible ending or even endings ); it means they can give me clearer advice and let me know if I'm being too obvious (tone it down!) or obscure (WHAT was that all about?).
I still don't like people looking at my unfinished work (and believe me, this prep is waaaay outa my comfort zone!) cos it makes me feel naked. But personally, I'd prefer a few close friends see me undressed, than go out thinking I'm dressed & have a stranger say my flies are undone