I have had this problem for a while. Designed many applications with a sidebar navigation but have never fully concluded on how to best use it within the grid.
just included using up a few columns, messes center of rest of grid due to outside margins.
B, outside the grid
not included in grid, actually adds to width (I could always change the grid to keep same width if this is the case)
C, squishing the grid
squishing the grid; the columns and gutter get smaller. showing this one in case sidebar is collapsible.