I've recently been using Inkscape for schematics / block
diagrams, which involved a lot of grid work.

It sounds like you are referring to the "Snap distance"
field on the "Grid" tab of the Document Preferences.
As far as I have found, this value is the maximum distance
the cursor must be from a grid point before it will snap
there.

e.g. if you have a 1x1mm grid, and snap distance is set to
0.25mm, then if you have the cursor in the centre of that
grid square, it will not snap anywhere.

To make sure the cursor will always snap to the nearest grid
point, snap distance should be at half of the grid spacing
value. (It doesn't seem to make any difference if it is
larger, so I just set all three values to the same).


Also, for schematic work, I have found that it is best to
turn off the "Snap bounding boxes to grid" option, because
with this on, horizontal and vertical lines will snap in
three ways: first one edge will snap to the grid, then its
centre line, and then the other edge.

Ken