Mr Acorn has just insisted that I put on record that the table slopes ever so slightly - you can't tell from looking at it - because the front and back of the machine bed are not quite the same level, and his table runs smoothly across the joins. I hope that makes sense!
He did this by angling the base of the legs very, very slightly.
It may not be necessary on other machines, if the machine bed is level (mine is a Husqvarna Viking Topaz 25, for the record.)