They do not need to worry about a rotation because all transformation will be added after the path creating:
GraphicsPath path = RoundedRectangle.Create(20, 20, 100, 100, 15);
e.Graphics.FillPath(Brushes.Blue, path);
Matrix matrix = new Matrix();
matrix.RotateAt(45, new Point(70,70));
path.Transform(matrix);
e.Graphics.FillPath(Brushes.Blue, path);