A setting on Point of Vision for using each of the types separately.
This comes up when I try to use walls narrower than one grid unit wide. Where the character isn't able to see properly through open windows or intentionally tight fit spaces, etc.
Perhaps something like 3 drop downs, with Defaults on leftmost option:
True Center: [Yes, No]
Corners: [None, Top Left, Top Right, Bottom Left, Bottom Right, All]
Edge Centers: [None, Top, Right, Bottom, Left, All]