When your gridded widgets don't take up all the space in a window, the grid anchor command allows you to say how they are arranged within the overall space.
Is there a way to change this so that the default revert to -anchor center instead of the 8.5 default of nw? Yes, there is: catch {grid anchor $path center} before all your grid calls. Yes, this can take ages. No, there is no other way. See TK bug 2062394 [L1 ] for reference.