If the P+L is positive a USD balance then I see no point in converting it to CAD or whatever currency the account is in, unless you think the USD will go down. If you are negative USD I assume you will be charge interest on the negative balance and only collect interest on anything above $10,000 USD in your home currency.
They have 50:1 margin right? So if i put in an order for Sell 100,000 worth of dollars and buy x number of CAD, then I should only see a 2,000 decrease in available funds?