It depends whether the CBM step is initiating turn. If it is, then the foot placement is usually very slightly to the side, rather than straight back (but this shouldn't be overdone), and with the toe turned in, the foot will, inevitably, make a curved track as it moves from one position to another.
Where there is no turn (such as a Foxtrot Three Step), while there is slight CBM on Step 1, the foot should go straight back.