I could be wrong but it seems to me there's a cost every time currency is converted. With that in mind I tend to withdraw J's from the ATM. That way its a straight one time conversion form Canadian to Jamaican dollars which I assume is at the best rate possible. I've never really sat down to figure out if it actually is the best method though. I do it mainly for convenience. It's a bonus if it's also the best rate.
Has anyone crunched the numbers?