We rarely use boxes during prewires for tvs. We typically run our lv wires (cat6, cat5e,etc,) and a Carlon conduit inside the wall. The Carlon usually high or low. This allows getting a fish tape up or down to where we end up cutting in a retro lv ring.
We have the sparky run a whip inside the wall with plenty of slack. We also will cut in a retro electrical box for power during mounting.
This takes time but allows us to mount the tv wherever its wanted and not have to modify the back plate or end up cutting in a retro ring because the wires are in the wrong spot