(I realize this is not exactly a plumbing question, but perhaps someone here has dealt with this issue.)

I'm installing a Kohler Symbio shower receptor (a corner installation). The directions say to shim out the studs next to the receptor one-quarter inch to allow the cement board to overlap the receptor. This sounded fine until I was about to start the construction. Where does the shim end? Do I have to shim out the entire wall, not only to the ceiling, but laterally as well? If not, how do I get rid of the quarter inch difference.

If I had realized this in advance, I would have notched the studs, but the drain is already in place and does not have a quarter inch flexibility.

The shim ends on top of the receptor edge that is against the wall so when the rock board comes down the wall it doesn't hit the edge of the receptor and cause it to bulge at that point.
You can shim the whole stud length or you can use a tapered shim. If you use a tapered shim, (cedar shake shingles work better than door shims, or you can cut your own) make sure the shim is at least 12" or longer.