I'm not qualified to answer this with any professional knowledge, but to my understanding food grade would just mean they've added some kind of an additive or done something to make it function for a specific type of purpose. Let's say, keep a certain form for example. But it's still sodium nitrite, 99%. It's still going to dissolve into water, and it's still going to damage you and kill you eventually. This would be my understanding, but I'm not an expert by any means.
So, I imagine it does not matter. You're just looking for +95% sodium nitrite, dissolved into water or something.