I don't add Cure #1, but I suppose you could. My normal curing times are from 4 to 7 hours depending on the thickness of the fillet, and I leave the skin-on whenever possible, so the curing is from one side only. That said, during this time, the salt does diffuse into the flesh, so I would think that Cure #1 would also penetrate the flesh. I would also think that even though the fillet is rinsed it gets an overnight rest.... so, would any salts and Cure #1 that had penetrated the flesh still be mobile and equalize throughout the full thickness? I think yes.
The reasons I don't add cure #1 is that the amount of my salt/sugar cure is not measured, it's an eyeball kind of thing. And, I control my smoking times to a maximum of 4 to 5 hours, shooting for a final internal temp is150°. My preferred smoker is my Big Chief and it runs about 175° +/-. Usually after 2.5 hours I'm in the 125°'s internal and getting some nice color, so I know the surface temp is over 140°. During the 4th hour my goal is to reach 140° internal and if that does not happen I transfer the fillets into my oven (using convection) and finish them to temp.