[noylj]

Many people find that they get best accuracy by seating the wadcutter out to the crimp groove or first lube groove. Also, there have been several reports that best accuracy with cast wadcutters comes with ONLY the lower lube groove filled.
The "problem" with your question is no one can tell you what your gun will prefer.
If you shoot HBWC, be sure to keep velocity under 800fps and seat just below flush and slightly roll crimp the case mouth.
For lead bullets, I have always found that best results are with bullets that are AT LEAST 0.001" over groove diameter and are a tight slip fit in the cylinder's throat.
You may need a larger bullet (and larger throats).
If you know where the leading is, you can diagnose the most probable reason for the problem.
A clue to what is causing the leading is where the leading first begins to appear.
If it appears near the chamber, chances are that bullet diameter or hardness are the cause. A diameter too small or an alloy too hard will allow high pressure gas to leak past the bullet, which erodes the bullet and leaves leading near the chamber.
If the leading first appears on the leading edge of the rifling (if you imagine the bullet being pushed through the barrel, you will note that one edge of the rifling does most of the work of imparting a spin to the bullet. This is the edge you see when you look through the barrel from the breech end), the bullet might be too soft or the velocity too high.
If the leading appears in the second (front) half of the barrel, the bullet is running out of lube.