I just mounted the switch in my dash near the headlight knob. The compressor does hold a tiny amount of air, but it’s negligible. When you first turn it on it will run for like 1 second to fill itself up. When you finish using it after putting air in tires or something, you need to turn it off, then squeeze the gauge trigger to bleed the air out of it before you disconnect the air hose or the hose fitting will shoot out of the compressor when you try to release it.
Also, maybe it isn’t a big deal, but I was worried about mud or dirt or water or just any junk getting in the compressor connector since it’s facing up, so I just took an extra end fitting and sealed it with foil tape and leave that in the compressor to keep anything from getting in there. You might be able to see it in the pic I posted.