I don't use tuff-cloth on fixed blades. I actually don't use it at all, as I haven't had the best of luck with it. That may be because I live in Houston and it's always either damp or scorching hot.
I've got two products that have worked best for me.
For fixed blades, it's Sheath, by Birchwood Casey. As I understand, it's silicone-based. Depending on usage (I have too many, most are now safequeeens), I coat the blade and any exposed portions with the spray, at least once a year. Then I blow out the sheath with canned air (get rid of the dust - both dirt and kydex) and treat the sheath with it, as well. On fixed blades, I see less rust and less "kydex scratches". I really like this product and get great results with it, especially on bead-blasted finishes.
For folders, I use militec. You have to re-apply every few weeks, depending on your use and the temp, but it works well for me. It works even better on my guns, though.
Just my experience, but if it helps, use it.