Indeed. I believe that's one big reason for why they have them available in the online account, so you never have to worry about not having your documents (a lot of letters do go missing in the mail)- it's so much safer to have them there, and very convenient to be able to just print and go.
I got my oath letter in hand at my interview, the IO just printed it on regular printer paper (same as my "recommended for approval" letter), exactly the same as the oath letter that was uploaded in my online account. My oath was 4 business days after my interview so there was no time to wait for anything in the mail anyway.