They don't have to be.. if your buying het pieds ask to see pick of the parents. If a breeder has Hypos, and Pieds for sale on the same table as the Hets you can bet the hets are good to go.
Now if I go to a table and there's tons of normals all labeled het this and het that with no visual recessives on the table and they aren't offering papers right then and there. I'll pass
Oh and NEVER take the I'll mail the papers to you as an excuse..