You need a filler, and you need to account for the additional depth that finishing off the rear of the cabinet will require. That will make that box thicker than the box that it is placed adjacent to.

Cook, you sure? It seems to me that the cabinets next to the 30" cab with the finished back toward the kitchen, will also need a finish panel on their backs which are facing into the adjoining room, no?

Like this?

pippabean and Buehl - yes - this is exactly what I was thinking, thank you! And I know there will be some futzing with the exact measurements to accommodate, but I just wanted to make sure I wasn't missing some obvious red flag. My brain turns to absolute mush in the heat....

Buehl, I will do a deeper dive in the linked thread. This is helpful, thanks.

Fillers or extended stiles are always needed against walls and in corners.

That's a clever way to utilize filler space, but you still need to cover the ugly backside of the cabinet.

Zmith - we will be accounting for that.