In my opinion all w/c snakes should have a fecal float done. Not to sure about the false negative thing. If the tech. knows what they are doing there shouldn't be an issue. Many of the parasites in reptiles are common to mammals so the techs are used to seeing them under the microscope.
I know that many wild snakes live with parasites but removing them in captivity protects any other (c/b or w/c) from getting the parasites. It also eliminates the chance the parasites will be an issue later in life or at a time of weakness in the snake.