Thank you Cameroo, you seem to understand.
Hedgeapple, one reason that I will round a belly is because it creates visually pleasing lines when done correctly. That said, some woods will tolerate a rounded belly more than others. Compression strong woods such as yew and osage lend themselves very well to a rounded belly. Other woods such as ash and serviceberry should generally be left flat. A practical reason for a rounded belly is to direct compression forces away from the edges which is useful if there are knots along the edges that might otherwise be a problem. Another is that it is easier to reduce weight without changing the tiller when the belly is rounded by scraping along the outer edges. But in the end, the main reason that I do it is because I can.