It thoroughly depends on the cell lines. The heat inactivation of Fetal Bovine Serum is unnecessary for most of the cell lines. It has been proven to us that the differentiated cell line does much better in heat-inactivated serum.
The best way to find out whether heat inactivation is necessary for your cell culture is to empirically determine that in your own study.
Mycoplasma and certain viruses are essentially non-infectious after heat inactivation. But whether it is needed or not, you must keep the experiments consistent and do it for all your experiments as still a lot is unknown about FBS contents.