I did this once when I was 10 and peeked at the end of my copy of "Around the World in 80 Days", discovered the ending, and swore never to do it again!
Books I could have ruined by peeking at the ending:
Charlotte Brontë – Jane Eyre
Anything adventurous – Clive Cussler, Dan Brown, Andy Berenson…
Stieg Larsson's "The Girl…" trilogy
Ian McEwan – Atonement
Penelope Fitzgerald – The Bookshop
Books where the ending is blindingly predictable and/or uninteresting:
Anne Brontë – The Tenant of Wildfell Hall
Emily Brontë – Wuthering Heights
Vanessa Farquharson – Sleeping Naked is Green
Penelope Lively – Moon Tiger
Ian McEwan – Solar
So as you can see, there is a fair split between "ending is crucial" and "ending is not a big deal" – but I still don't want to spoil the book!