The MinIMU-9 v5's mounting hole is not electrically connected to anything on the board, so you can use that as a way to fix the board to your robot. You should generally be cautious when mounting electronics to conductive metal surfaces; just make sure none of the header pins or conductive parts of the components could come into contact with the surface of the bare chassis.
Assuming you have a rigid chassis, the exact placement of the MinIMU should not affect the gyroscope readings. If you place the MinIMU at the center of rotation of your robot, you can minimize how much rotation adds to your accelerometer readings. As for the compass, you should try to keep the board away from stray magnetic fields (e.g. from things like wires supplying power to the robot's motors).
We have an Arduino library for the MinIMU's LIS3MDL compass that includes a calibration sketch, "Calibrate.ino". You can find a link to the GitHub page for that library under the "Resources" tab of the MinIMU-9 v5's product page. The calibration routine requires that the compass be rotated in all directions while the sketch is running. For me, it helps to imagine I have to paint the entire inside of a basketball with the tip of the MinIMU.