I am new to tang sights and I may be looking at this wrong but here goes!
I am putting a MVA tang sight on my Shiloh Sharps 1874.  I know some tang sights need to be shimmed from left or right to line them up with the front sight. 
After leveling the barrel do I need to shim the tang sight base to make the staff plumb / 90 degrees from the level barrel?  Other wise one would be looking through the aperture hole at an angle, thereby closing the hole some.  Am I being to critical?
Thanks

Shooting gongs is does not take less accuracy than shooting off a bench at 100 or 200yds. I'm getting a little tired of this constant comment every time the subject of accuracy comes up!
1 moa is 1" at 100 yds. So at 1000 yds., it's 10" and what happens to a bullet traveling through the atmosphere at 100 yds. or 200 yds. has far less effect on the bullet, than what happens to the same bullet traveling to a 10 moa target at 1000 yds. with the bullet exiting over the speed of sound, dropping back through it, and at an arc that's 60 ft. above the ground and all those wind flags people shooting 200 yds. use.
If some person with zero knowledge of shooting past 200 yds. from the bench thinks that hitting a target at 1000 yds. takes less accuracy or skill then they need to test themselves by trying it, and then tell everybody how easy it is, or how little sight adjustments mean at 1000 yds.

Thanks for all the good information.  This is going to be a learning cure for me.
One other question, how do you adjust the staff?  I can lean it forward or back.  It kind of snaps to the upright position in between.

 
There are no scoring rings on a gong. A hit on a gong in a gong match counts the same whether it hits the edge by a half bullet or the center. Target matches don't work that way...