Its like you say, blacking on the base plate is likely to scrape off, especially in shallow canals, so what most boat yards recommend is to just do the sides. What protects the base plate is usually the crud that has built up over the years so if you scrape this off to apply bitumen and then touch the bottom you will be exposing new steel to begin rusting.
The reason the sides are done is beacuse they will tend to rust more quickly, especially at the water line. This is because at the water line the steel is constantly getting wet, then drying, with plenty of oxygen about, all of this just promotes the rust. Also the importance of the sides being done more than the base plate is beacuse they are usually 6mm, whereas the base plate will be 10mm, so will rust through quicker if not painted.