Spring loading is NOT needed and actually a negative bias spring (canceling out some of the weight of the torch and cables) is needed if you cut thin material. Any downward deflection of the material will throw off your pierce height.
If you are paranoid about the Z going to far just mount a second switch above the Z home and make it a limit to stop all movement. I have been running a table with a floating head for over 5 years and I have always been fast enough to stop it from destroying anything during a home (touch-off) move. You just need to keep the slide for the floating head clean and lubricated. Mechanical switches are very reliable.