Does the position in which you mount an IMU matter? Forgive me, I'm sure this is a total noob question, but I have never seen it actually addressed in any example or demo. 

When using something like a MinIMU-9 board (https://www.pololu.com/product/2738/), does it matter what position you mount the board on your project?  I mean, does it have to be positioned so the proper axes line up with the axes of your vehicle, or is there some calibration to set that alignment?

For a ground vehicle where I will concerned primarily with only one axis, are all axes interchangeable?
If the axes of measurement don't line up with the axes of motion, then comparing the IMU outputs to vehicle-relative (VR) coordinates will require some vector arithmetic to convert one to the other. It can be done, but requires some engineering time to code it and processor time to run.

It wasn't clear to me at a quick read whether your chosen IMU can do the coordinate translation itself, or your MCU has to do it. In either case, the computation will always reduce the accuracy of the original data, maybe a lot, maybe inconsequential, but expect some degradation.

Bottom line is: if you can line them up, things get simpler.