Pete,

The CVT in the synergy hybrid drive is a simple set of planetary gears that function similarly to the differential in a normal car. The use of the "B" setting when going down a long grade is unlikely to cause excessive wear or damage. It will cause your ICE to work harder so there is a downside there (but minimal). The largest negative is some lost MPG using the "B" option, so that's a reason to minimize its use.

JeffD