Based on your experiment you can whether deactivate it or not.for instance, for experiments that include immune molecules it's necessary to deactivate FBS in order to prevent any intervention. But for any other type of test which immune molecules are not involved, it is not a must! You can look up the "FBS deactivation" in ATCC guide for animal cell culture.
This strongly depends on your experimental setting. It is not necessary for simple cell culture, but needed to avoid stimulation from FBS possibly affecting specific experiments (e.g. stimulation with growth factors etc.).