As you've said, 'B' mode ramps up the regenerative braking which can improve fuel economy and save wear on the brakes. We'd recommend using this when you're driving in heavy traffic around town or travelling down a steep hill. At higher speeds, it's better to stick with 'D' mode and let the car coast when you lift off the accelerator. You don't want it slowing down of its own accord at motorway speeds. You'll soon learn the best times to use each different mode but, if not, stick with whichever feels the most natural to you. Neither will cause damage.