If it is a US dollar credit card you should be charged in US dollars. For me, the issue is paying the bill, since I have no source of US dollars and would have to convert CAD dollars at fairly high rates. (These would generally be small amounts; if larger purchases I might consider keeping a reasonable amount of money is a US dollar account and replenish it when rates seem good.) However, for me, it is a lot of worry; it makes more sense for the amounts I generally spend to have a Canadian credit card with no foreign transaction fee.