I never knew you could do this. The JW SFO Union Square just sent me a welcome email with this info and so I searched here and of course there is this thread. I'm in my account all the time on the website but this feature is pretty hidden. I do get my folio emailed to me but this is a nice alternative.
As a Marriott Rewards® member, you can access your
hotel bills online – from wherever you are – through
your online Marriott Rewards account. Electronic bills
are available for your stays at over 3,100 participating
Marriott® and The Ritz-Carlton® hotels throughout
the U.S. and Canada, 48 to 72 hours after checkout.
Your hotel bills will remain accessible from your
Marriott Rewards Account Activity page for up to
one year, depending on the brand.