I am going through the board's calibration process and it is declaring success. Do I still need to pay attention to how I mount the board, that is, match the x,y,z directions printed on the board?
Not necessarily, so long you're aware how the IMU's mounting orientation may affect readings.
I don't know what IMU model you're using or how, but assuming it integrates a gyroscope and accelerometer, and you're using readings from both to compute roll, pitch and yaw, you must be aware how results will be affected by the device's orientation relative to its host device (e.g. a robot) and the Earth's surface.
For example, if you mount the IMU at an angle, this will result in non-zero roll and pitch readings; if it's upside-down, this will reverse the direction of yaw changes. Depending on the intended application, you'll have to account for that in code.
In general it's worth the trouble avoiding these complications and just mounting the IMU in a way that agrees with the host device's orientation conventions. However you're free to do otherwise, so long you know what you're doing.