Convexity of the finished stone will be a benefit for two reasons, the first being that water stones dish by their nature and the second is that it will make it much easier to make a flat bevel.
Technique weighs into this considerably.
I see no harm in wanting things flat, after all I like to pursue tight tolerances myself, but there is a workable reality that must also be considered. A stone simply won't stay super flat in the process of using it, but it can be maintained fairly flat if you sharpen using the whole stone.
Bumbling forward into the unknown.