Ah, but do you know which burns more fuel the higher CI or the below optimum altitude?
Different answers for different aircraft......
Gathering the right information about your aircraft/engine/fms configuration and the airline cost structure and CI will help you make better decisions.
The problem is that CI is so poorly understood and rarely taught that facts are few and far between and rumour, urban legend and out and out wild guesses prevail rather than a fact based decision process.