Search for GRIDMODE and SNAPMODE in your (AutoCAD) Help,
there you can read the values are stored in the FILE(Drawing).
Are you using a dwg/dxf-template in Inventor? Then you have to edit this template file
<sorry, i am not an inventor user, so i don't know how the exports words on this side>
or
Are you using a dwg/dxf-"Setup file", an INI File or anything like this? Then you have to edit this setting there.
I remember "exportdxf.ini" ??
I don't know if you can set a variable directly in this ini File (and i can't find the documentation),
but you able to use a template (it looks like that) -> AUTOCAD TEMPLATE=
Back to a template file: Set your variables there.