I just tested it on my Kindle KB, 3.4. Set it to ON it shows publishers font (Forgotten Garden by Kate Morton).
I went to home and back and it is still ON.
Went back to Home opened another book without publishers font. Looked at font setting. Went back to home, opened the first book again and it still shows ON.
I have another book with publisher font activated. Changed back and forth and always the first book stayed like I set it in ON position.