The photo ops are paid in advance via PayPal, so currency exchange will take place through the PayPal system.
The autograph sessions are paid in cash at the guests tables (unless they start having credit card machines at the tables - which, with the advent of smarphones and programs like Square, they might). Some guests wish to be paid in USD, some in CAD. Those are listed in the first post of this thread.
If you're travelling to the convention from outside North America, it would likely be beneficial to have both currencies on hand. Or make good friends with someone from the US, bring your CAD, and make an equitable cash trade at the convention. =)
There used to be a currency exchange around the corner from the hotel (I used to cash my traveller's cheques there) - anyone know if it's still there? I assume it would be, it was a good location surrounded by hotels.