Actually, this is not a yes or no as to whether ot not you grind before or after heat treating. Each has it's advantages. If you heat treat before grinding the grinding will go faster and you won't have to worry about overtempering the steel. If you heat treat before they the belts will cut more slowly and you will have to cool the blades more often, slowing things more, but you will not have to worry as much about warping.
I don't use foil for heat treatment, I prefer anti-scale compound, but you can unwrap before quenching. You would have to do it very quickly with something like pickup tongs while still in the heat treating forge or oven and then move quickly to the quenchant.
Doug