You must select flush-to-zero mode if all the following are true:
IEEE 754 compliance is not a requirement for your system
the algorithms you are using are such that they sometimes generate denormalized numbers
your system uses support code to handle denormalized numbers
the algorithms you are using do not depend for their accuracy on the preservation of denormalized numbers
the algorithms you are using do not generate frequent exceptions as a result of replacing denormalized numbers with 0.
You can change between flush-to-zero and normal mode at any time, if different parts of your code have different requirements. Numbers already in registers are not affected by changing mode.