"Citation needed" I think in general, PCBs are likely to be in any position. Considering the millions of PC that have a motherboard that is vertical, and mobile phones that are held in various positions, the idea that PCBs are only ever horizontal is quite tenuous.
The front of a book is always the front, regardless of whether the book is laying flat or not. It can still make sense to talk about the Front or Back of PCB.
In one of our products, it was deciding to mount the PCB upside down with regard to other products. We did not rename the PCB layers because of that...
Anyway, you are free to call the layers whatever you want!