Have always used blank ones... Actually, I have found the prestamped z-tags to be just about worthless in 3 years as far as being able to read the tags while the ink ones are still going strong.
We have a whole set of 1-about 50 of the yellow with black numbers on em (allflex) that you can't read unless you got em in the chute and can get a hand on the tag. So this next go round they will probably all get new tags. I hate not bein able to read a cow tag when I'm drivin around, or walkin thru the herd.