Bell is reported to work 'till about 200 miles south but.... YYMV and as John says, it may vary by the particular states proximity to the border Shaw works quite well though. There have been some reports with the Shaw systems equipped with auto satellite finding not working too well. They seem to have a GPS type system built-in so they know exactly where they are and can be cut out as a result. (Just as an FYI, you should be aware your services agreements forbid using either system below the border!!)
Others often use the Dish system which have a "pay as you go" system available, campground cable, or streaming (very expensive for Canucks).