I was thinking of building a bench in MDF. Do I need to countersink the screws or will they go down a little automatically if I use the power driver (so I can fill the surface smooth afterwards)?
It depends on the type of screw you plan to use. Self-drilling chipboard screws from a good brand, such as Essve, work fine without pre-drilling. If you want to use torx bits instead of PH2, a countersunk universal screw, such as Essve 4.0x40 mm, works excellently. Feel free to try screwing into a scrap piece of MDF against a piece of timber before you "go live." Good luck!