The subject pretty much says it all.
10 years ago I built my first pair of bookcases, which launched me into this hobby. And I thought I was smart, and put a notch at the bottom/back to go around the baseboard... two years later we moved into a house with taller baseboard.
I am just now planning a tall/narrow child's bookcase and I wasn't planning on notching the back at all. As I put my mind back to various plans or photos that I've seen over the years, I can't recall seeing many, if any, custom units that are notched.
The reason I ask, is that my Mum asked me if I was going to fasten it to the wall for safety. (She doesn't want her grandkids pulling it over...) This made me think about notching again, in order to get the bookcase snug up against the wall. Hmm.