Hi, as far as i am concerned, the climb is at constant IAS and then, you will change to constant MACH, climbing at a constant IAS will result in a MACH increase upon reaching a level in which you will change to constant MACH, as you may know from instrumentation a climb with constant mach will result in a reduction of IAS/TAS/EAS/CAS...
Some reasons not to fly at the optimum altitude may be more headwind or more tailwind at other altitude. Ant the reason of being above, below or at the optimum altitude is just fuel saving. I dont know if that makes sense for you or answer anything you were asking/wondering.