I have just started cleaning my begonia tubers , and thought the attached photo might show what advantage there is if you bury the tubers instead of leaving them on the surface as used to be done. It was thought that you should keep the surface of the tuber above the soil to avoid rotting when water could collect in the hollow top but in fact if you have soil in and above this it draws any water out. There is however a real benefit from covering the tubers, it is possible for the whole surface of the tuber to form roots if in contact with soil and you can see how much root you can get doing this. Blackmore and Langdon say it is essential to bury them.
(http://img362.imageshack.us/img362/741/1000431zs4.jpg) (http://imageshack.us)
I've done this for several years, too. Another advantage is that the big plants are simply more stable.
It surprises me when I get so many people saying you must not put any soil on the top, I even had a "garden expert" on the Radio say I was completely wrong, to cover the tubers but I have done it for around 20 years and won many trophies.
The only reason I can think of why anyone would leave a tuber like that on the surface is if it was prone to rotting. Begonias aren't my thing; is this the case?
It was the fear of rotting that made people start to leave them uncovered but it has now been realized that covering them stops them rotting rather than starts it.