My 25" Oliver jointer / planer uses torx head screws on the inserts. There is no way that I would want to replace them with flat head screws! For starters, the torx head will allow much greater torque w/o stripping out the screw head.
On my Oliver, it's an all day job to remove inserts, clean the heads, and reinstall on a new face. I've tried different brands of inserts in the past and the inexpensive ones tend to both break and dull easier than the higher quality ones.