Bicycle Mechanics - 2 cleat questions (crank bros)

when is it time to replace crank bros cleats? mine are getting pretty worn down, but they still clip and i can't pull my foot straight out without twisting. is there a mark or point i should be looking for?

also, i had gone to just replace them anyway, and one of the screws is stuck and stripped. i have doused it in tri-flow several times and tried wiggling the cleat back and forth with pliers (with the other screw out) but since these are MTB shoes i can't spin it all the way around. the allen thread is rounded out to the point where i can't get any traction on it. any ideas? thanks!

With normal allen you can try hammering a slightly larger allen wrench into the
stripped hole and hope it grabs enough, but with shoes there is usually too much
give to allow the wrench to go in far enough. You could drill out the screw with
a drill just smaller than the threading size or just grind away at the cleat and
allen head with a grinder til the head is ground away and the cleat comes off.
Rest of the screw is usually not hard to remove.

you can drill the head out of the bolt. Then its pretty easy to get the threaded part of the bolt out... Unless the bolt has corroded to the cleat plate (did you remember to grease the threads before hand?)

You can use them until your foot slips out of the pedal without trying. When it becomes time to replace them, you can use a hacksaw and cut a groove for a screwdriver in the cleat and screw and then just unscrew the screw using a screwdriver.