So, you aim to measure orientation with respect to earth's z-axis? Meaning, you do not care about the z-direction (what a compass would read out to you)?
When accelerometers are steady, they make perfect orientation sensors for this purpose. If you need the z-direction, you could add a compass module.
But this method is not good when the object is moving back/forth (ex. walking). But you can add a gyro sensor and create a nice filtered system for orientation sensing (getting rid of the compass in this case).