> What would be the best way of forcing the path to (always) relative coordinates?
Changing the source and rebuilding. Actually it shouldn't be that hard
to force Inkscape to always choose relative coordinates, but why would
you want to? The current behaviour is that Inkscape minimizes the length
of the path data.

As for why you would want to - I can give at least one answer to that. I was recently using Inkscape to create SVG images for use in a little shoot-em-up game I was writing. Ultimately I didn't want the complete Inkscape file, I just wanted to extract the SVG for each sprite then move or position them manually.

Relative coordinates mean that it's just the initial move (m) command that needs to change, and everything else moves with it. It makes manually consuming fragments of an SVG file in another context (an HTML5 page in my case) a lot easier.