Good one.
I have also people mentioning that it needs to stamped and signed by bank. I don't find any such requirement in CIC website. Not sure if people created that 'sign & seal' for themselves. This is the requirement mentioned in CIC website:
The letter(s) must be printed on the letterhead of the financial institution and must include the applicant’s name, the contact information of the financial institution (address, telephone number and email address), the account numbers, the date each account was opened and the current balance of each account as well as the average balance for the past six months.
http://www.cic.gc.ca/english/resources/tools/perm/express/intake-complete.asp
I feel one should be good if the bank documents cover all the above info in some way or other.
I know someone who transferred all the funds to a new bank account just because the new bank gave the letter sealed and stamped. Not sure if that was a overkill or necessity!