Back in the 70's my wife picked up a cap fragment in the lid of here left eye lid. It was there, a small green spot until about mid 1990's, when it finally came to the surface and came out - no scar, even. I've worn some sort of protection since about 1976 or so, generally shooting glasses before that, date but not always until Tracy picked up that frag. Now, I were perscription glasses due to a requirement to see well so always have protection nowadays automatically. That they aren't period correct, doesn't bother me any more than being politically correct.
I cannot imagine being on a firing line with cheap cap locks or flinters and not wearing some sort of protection. Neither of my cap locks throw fragments as they have properly locks with good leaf springs. There are enough coil springs out there throwing cap fragments, along with flinters throwing hot fouling sideways, that wearing glasses is a must.
Those with rifles having coil springs and buy replacement locks having leaf springs from outfits like Trackofthewolf. Minor inletting is required.
In my most humble opinion, of course.