They will crush you. FCMs can roll/covert the exposure at any time then assign to you the worst possible price of the day. Anything within the days range. They pocket the differential. This can be a sizeable chunk of change when you net the FCM company wide exposure from all of their customers.
Another reason to trade forex through IB or a broker that makes the rollover purely a financing issue. With IB, there is no rollover. Just interest earned/charged based on your cash position in each currency at 5pm Eastern Time.