Sensing distance is one thing, but the other thing to watch is the hysteresis - difference between switch-on and switch-off points. I had problems with my machine where I homed Z, then X and Y together. However, because of the tiny difference between on/off switch points, the vibration during X/Y homing was enough to trigger the Z home switch which by this time had become a limit switch. Fortunately, there is a Mach3 setting which offsets the home position slightly after homing which avoided the problem, but it's something to watch for. Spec sheets for more expensive switches seem to suggest something like 10% of the switching distance but mine was rather less than that.