Depends on which grbl hex you are using. If you are using the jtech grbl 0.9g then you can use S254 (high power) and S0 (off). If you are using the 0.9j grbl then you can use S1100 (high power- perhaps even 1200) or S0 (off). I use the picengrave pro 5 for most of my work but I have also used easel (changed the z codes to the s codes) or the Inkscape Jtech extension for gcode generation. In the cases of Easel and Inkscape, I was not going for shading of any kind. It was a pure on and off (or full power and off) type of signal I was looking for.
After some experimentation with both grbl hex codes, I came up with a preference for the 0.9g version (less latency) and created two separate power control boxes - one for laser (0.9g) and one for spindle (0.9j) operations. That may not be ideal for you but it made it easier for me. I switch over the X, Y, Z connectors that I set up for each box as well as the USB cable. I leave the laser and spindle connectors in place. I have long gone away from trying to deal with the limit/homing switches as they were problematic for me.