You can deadhead it for appearance sake, but it's really not necessary as it would be for rhododendrons that produce bilions of seeds and ugly seed pods if spent flowers are not removed. In fact, I'm not sure if Stewartsonia even produces seeds. Deadheading would prevent the ugly brown stage of faded flowers, but most people find the process pretty tedious.

Most people shear them after blooming in to unsightly balls, cushions, or boxes. I like to plant them where they have room and let them find their own canopy, preferably with other azaleas.......even Stewartsonian. Yeah, they look gawky for a few years, but eventually the will grow into pleasing informal shapes.
Here's one of mine.
Mike...near Seattle