All of the above comments are a good summary of why I avoid using flat head screws at all if any other options are available. When you think about it, the whole reason that clearance holes are larger then the screws that go through them is that usually the precise location of the tapped holes is somewhat inaccurate. The size of the clearance hole allows the final screw position to align with the tapped hole and still hold the untapped piece tightly. BUT... cone-shaped countersinks create a very specific centerline location for the screw. In order to function properly the screw must have contact all around the countersink. The centerline of the countersink and the centerline of the tapped hole are NEVER in perfect alignment, which means you are always inducing some lateral load on the screw. It also means that the contact force of the screw on the countersink hole will vary widely around its circumference. If those screws are in a pattern, once one screw in the pattern is tightened all the others will be slightly off position. We're talking thousandths of an inch here, but it might as well be feet. That misalignment can also cause the screws to gall up in the hole, making their removal very difficult. Experienced machinists will tell you that the most common type of screw to have to drill out are flat heads. Bottom line? Find another way. If the presence of bolt heads is a problem use counterbored holes, or maybe use button head screws.
