BMO has American dollar accounts[1]. That way, you can convert only what you need. Or, when you feel is the best time due to rate fluctuations and so on.
Additionally, if you ever travel and spend money in the US, you can spend in US currency, again avoiding exchange rates.
Additionally, if you ever travel and spend money in the US, you can spend in US currency, again avoiding exchange rates.
[1]: http://www.bmo.com/home/personal/banking/bank-accounts/us-do...