What service conditions would require the use of a bottom slope of the floating roof pontoon instead of a flat bottom? I have seen both used in industry.
I think more a design issue than a service condition. If you adjust slope and depth of pontoons so buoyancy matches the weight distribution, it helps with design loading but complicates construction. With a flat bottom, you'll have a rolling moment, tending to roll the pontoons outward at the top, that must be accommodated in the design.
