I have been wiring line locks to the off button for years and yes you are correct, the reason I don't use the on button is it grounds that circuit when pressed. I used to also use a MSD rpm activated switch to turn on the up shift light, I used it as a shift light on my '85 GT. I tried to keep that car as stealthy as possible. I used to keep a 6700 rpm chip in the rpm activated switch.
If I buy a part or parts from you do not send me those parts through USPS, I would actualy like to get the parts I purchase.