The general answer to downshifting autotrans for engine braking is that it has been good practice (gas engines) but more recent innovations with computer controlled trans features make it wise to know the facts of your own vehicle's setup.
Diesel engines - another matter - engine compression retardation not much help. On my 1952 Mack with the big valve Cummins, the Jake brake (Jacobs exhaust brake) is a big plus. The 2003 Duramax is coupled with an Allison which has an inbuilt retarder. Very good. The 2001 Dodge Cummins is 6 speed manual and will give some egine braking in lowest gears - but trailer brakes are a necessity with that setup.
