the most recent thread I have seen this discussed is:
http://www.airforums.com/forum...ead.php?t=9362.
Andy at Inland RV says it's a no-no. The rivet head with the washer is also frowned upon by Andy because the washer will fail leaving no seal. I know from experience the washer failure will happen from dealing with metal building roofs where the washer under the screw head fails from the uv rays from the sun and the slight expansion and contraction of the roof sheets due to heating and cooling.