Assuming you have a room with walls available, place a square solid in the corner thus..

With the clip tool, clip the object in half like so..

Place a second solid like so, assuring that you remain on the grid lines, and that they extend to within the 1st solid..

Clip the second solid off, be sure you are on the gridline..

Clip off the second side, making sure that you follow the original path, this will ensure no gaps form between the two..

After placing a second solid and following the same steps as #5, you will end up with a group like so..

You can retexture any way you see fit, to end up with a fine round corner just like this

The arch is good for this too but like this method you can make different corners

Old school stuff still works, but I prefer to use the arch tool for wall corners. They butt directly to the wall ends, so the face splitting is limited to the floor and ceiling. You know you're lacking sleep when...you try to pull the wall vertices on the tutorial...lol.

No. Keep it a worldbrush. That way the backfaces are culled. Furthermore, leaves are convex regions of space, so subtracting a concave region from a leaf does not split it.