Fill flash

Would you use a stofen diffuser when using fill flash thanks...Ray

Whether you're using flash as fill or the primary light source doesn't matter.

What matters is whether the surroundings are conducive to blasting your flash through a piece of translucent plastic and throwing photons in every direction.

The Sto-Fen works best when there is a low ceiling or another close, large, white surface to bounce the light back into your field of view. It won't do you any good otherwise.