I've not found resources about growing sassafras leaf for gumbo file or tea. I harvested a few pounds in the spring, then decided to hold off and see if the flavor changed through the season.
Later in the season is more vegetative and bitter. As fall sets in the leaves look like crap and have fungus & worms. Of 20 branches I cut today, I only put one up for drying. The rest are compost fodder.