There are two places you apply coatings on pistons: skirts and crown. Stock pistons have skirt coatings, and you want that on your forged pistons as well. Swain tech skirt coating is rumored to be very good and robust, better than "stock" coatings e.g. by CP.
Crown coating should be used only in race applications. It keeps piston cooler, maybe "too cool" under regular low load driving. Even if it's not detrimental, it's really not needed.