It is common, but it's not correct.
What you're describing though, at about an eighth of a turn, is not as much as I've seen on some. It's not good, but it's not necessarily out of line with what you've done.
There are still a couple of things you can check out though, to make sure it's at least minimized and the system is working up to it's potential.
As pointed out, it's a physical property of the pitman arm follwing it's arc while steering.
To keep it to a minimum though, Ford designed it so that the point where the draglink mounts to the tierod sat at a modest upward-facing angle (let's say 40° up, just for discussion). And the steering box was located, and the pitman arm was sized, so that when it travels through it's arc it would also minimize the rotational pulling effect on the tierod.
If your tierod is rotated down (so that the mounting pad/boss is facing more forward), or your pitman arm is much longer or shorter than stock, you'll get more rod rotation than normal.
It's a slight problem with the popular Chevy TRO setup. Since the new passenger side rod end has the mounting boss for the draglink facing pretty much straight forward (since it was actually for a stabilizer mount), instead of angled slightly back/up, you get more rotation of the tierod.
What angle does the pad on your tierod ride at? Is it sitting at a slight upward angle to better align with the pitman arm?
When setting toe-in, it's possible to lock the adjusting sleeve down in such a position that the main rod is rotated farther down than it should be. You can see that not only in what angle the draglink mount sits at, but also if the passenger side rod end looks twisted instead of in a "neutral" position.
When that's the case, it's as simple as loosening the clamps and rotating the rod back into a more correct position.
That'll at least help, if it's part of the problem.
Even the length of the pitman arm might be aggravating things. Many dropped arms are "longer" as well, so the point of rotation of the upper draglink end is farther behind the plane of the tierod, which will cause more twisting still as it actually tries to rotate the tierod as it pulls the draglink back as well as to the side.
Hope you didn't go all glassy-eyed after the first forty three paragraphs.
Paul