I was surprised by the 05 Durango w/ 5.7 and five speed auto when we got it. It was the first vehicle that I had ever seen which, with cruise control on, down shifted automatically to reduce speed on downhill grades.
To the OP's question. Yes it is absolutely advisable to downshift the automatic transmission in down grades. It will not hurt the transmission, and on a long down grade will prevent brake failure.
Sometimes it is necessary to tap the gas pedal a little bit and raise engine RPM before the trans will actually shift into the lower selected gear. At least this is true on the older vehicles I have driven.
