1. It's utterly pointless to rejuvenate a Phillips screwdriver. Grinding the "sharp" part of the tip off means it will never fit properly in a Phillips-head screw. Filing the grooves and flats takes too long; and I strongly suspect that it's difficult to find a file that would produce the correct angles without considerable skill and patience.
2. The best screwdriver tips are forged, not ground.
3. I believe that the Phillips design was intended to cam-out at some given torque, but when it does, the screw and the driver are in danger of damage. Using a worn driver will endanger the recess in the screw head, using a driver on damaged screws endangers the driver tip. Any person having a lot of trouble with worn screwdrivers better do some inspection on the screws that are being driven; you'll probably find that the screws are as ruined as the driver.
4. My simple test for screw/driver wear is to take a NEW, PROPERLY-FORMED Phillips-head screw about an inch long, and insert the driver into the head as normal. The screw should not fall off the driver when the driver is held parallel to the ground. Similarly, a used screw can be tested by inserting a good driver into the recess, again the screw should hold-onto the driver when horizontal. Worn screws, or worn drivers will generally not "stick together" when held horizontally. Clearly, a magnetized driver or a magnetized screw isn't suitable for this test!