That is correct, with the opaque transfers, which are what go on the dark shirts, you leave just the way they are, because when you apply them you peel them off of the backing first and lay it on the garment face up, then cover it with parchment paper or a teflon sheet and press. So you print it exactly the way it looks.
With the light ones you do mirror them, because you lay it face down with the polymere on the shirt, and dont peel it till after you press. Does this make sense the way I explained it ? let me know if you understand.