It’s perfectly okay to use a corporate rate one is authorized to use. It’s usually best to just have the travel department or someone at the relevant company make the reservation on your behalf.

Definitely do not count on using a rate code without authorization. You will be charged rack rate if you are asked and cannot provide proof of eligibility for the rate.

In Asian markets, travel agencies that cater to local markets sometimes do have rates that are substantially discounted. So the rate parity thing is not actually always true, but usually is the case with FS. Speaking and reading a local language, or having a friend who does, can reveal special offers.