Doesn't 16 bit audio already require dithering to capture the full audible dynamic range? That makes me worry somewhat about cutting more bits and layering dithers on dithers.
You always want dithering no matter the bit depth, but 16-bit is actually quite a lot - there's literally no point in better than CD-quality audio. (Except of course that real life is in surround not stereo.)
But yes, if you're doing it repeatedly you'd want an un-dithering filter. Noise reduction tends to do this by accident but it helps if you know what the dither shape was.