I was reading one of the Clarence Lee Engine books (RCM reprints of his columns) and was amused to note that he was complaining of engine returns where the user had replaced the phillips head screws with cap screws. He claimed that people were now over-tightening the heads, causing the head to warp!
Probably some truth to this, especially now when most engine heads are held down by 4 screws, not 6 as most were in the "good old days". The key point with the OS is to get the correct screwdriver. I think is is the Japanese standard, not the US phillips standard which has a different angle in the flutes (or whatever they are called).