To take it to the extreme one wheel free spinning while one at stall would result in one ESC seeing the majority of the voltage (And promptly breaking itself).
This was along the lines of the issue I was thinking of, hence the original post though I was perhaps thinking about it from the wrong angle (I was concerned about the arrangement from an amperage perspective). So OK, that idea is out of the window. This leads me back to my question about rule 7.5 and wherever it applies to the whole robot or per battery pack when wired in series.