I engine brake just about everything I own (second, not usually first, though). This really helps if you've got the 9" binders. I've been doing this for over 20 years with various Chrysler and AMC (904 equipped also) products, and I've never burned up a trans. The only cars I've owned that I had to build transmissions for were screwed up when I got 'em, or beefed up for performance. My Duster had 245,000 miles on the original transmixer which was still shifting fine, I freshened it up when I hopped up the six as a matter of course and swapped in a low gearset. I can't give you the overall big picture whether engine braking with an auto is good or bad, but my experience is that it has had no ill effects, and is very helpful to my driving style. I usually keep the trans in 2 range for city driving until I've built up a good head of steam and do the 2-3 change manually. This way I don't have to wait around for the kickdown if I need the power to dust off some slug...
D/W