very safe, I am using is now;
TD waterhouse has a service called TD wash (they have mandrine service)

for your RRSP or TFSA (Tax Free Saveing Account) , due they only CAD account, once first time you buy US stock, when next time sell, you can call them do a <WASH> to hold your USD (do not exchange back)

and next time, when you buy some other USD stock, call them to wash out your USD money market , directly buy use USD $ amont, you can do that forever;

so you can use RRSP or TFSA account to buy US stock (USD $) constantly without losing any exchange

TDwaterhouse can help you to "wash" USD: 1)buy US stock with CND; 2)Next time when you sell it, call waterhouse the same day to let them buy USD money market fund with same USD value. 3) Next time, buy US stock again, and call TD again to sell your US money fund the same day. A little bit unconvinent, but at least can save you 4% rate loss on each trade-----I always play my RRSP acount (CND only) on US stock this way.