Valve grinding compound and, when that doesn't work, cutting a slot in the
screw head -- two good ideas. You may be able to avoid problems entirely by
using a top-quality, brand new phillips head screwdriver or bit. That will
assure that maximum torque is applied to the screw head.

For round-head or pan-head screws, there is one last, desperation move you
can try before drilling the screw out. Using a flat file or the grinding
wheel on a Dremel tool, CAREFULLY create two "flats" on the edges of the
screw head and use a vice-grip pliers to grasp them. This will allow you to
put much more torque on the head than you can with any screwdriver.

Needless to say, you should not reuse a screw that has ANY damage to the
head

-Elliott Drucker