I agree with Marko that tangential image is most important in breast RT. And my instinct says, yes we should correct on these images.
Important is that your primary goal is to treat your PTV and secondary to protect the heart. Based on this, most important is the correct position of your PTV (=breast) relative to the beam. Here chest wall is probably a good landmark. The shape of the breast might change under treatment, the chest wall rather not. The level of BH is not that important, it serves to protect the heart. I dont want to say it is not important at all, but less than a correct PTV positioning.
Our imaging procedure is a bit different. We first take an PA or AP kV-image and correct only for LAT and rotation (RTN) based on the chest wall. This is not that sensitive to the BH level. Then we take an MV tangential (in BH) and correct ONLY for VRT and LNG and PITCH in a sequential way (this is possible on Truebeam) if needed (the tolerance level depends on the margins the doctors were applying to their PTV), again based on chest wall. If a shift was applied, we take a new VRT at the same time.
THE perfect procdure probaly does not exist and depends anyhow strongly on the individual patient. Anyhow, hope this helps a bit.