I prefer ferro rods, also. I do a lot of "survival stuff," and haven't had any requests for mag blocks yet. Not that they're not good, just don't seem to be as popular. I prefer to just carry a little extra decent tinder in the "survival kit," myself. I'm sure you know this, Anthony, but about the only mag blocks worth messing with are the Doan brand. The rest, in my experience, seem to enjoy waiting for the worst possible moment to eject your ferro rod into a pile of leaves.
Sam
I'm with Sam on this one.
I make my own tinder packs which are Cotton balls, coated with Vaseline. They light when wet [well at least....always lit for ME when wet]. I also have charred punk wood, charred/raw fungus, charcloth and fatwood in my fire bundle. but I also agree with what's said above.....wet natural tinder is way harder to light with just a ferro rod....sometimes even with a lighter.
Even though I have my magnesium blocks in my pack, I never really use them. I certainly don't put them on my sheath. That should not detract you from offering the service as a sheath maker...just make a decent and 'generic' template that you can simply add to the design.