there was a good thread about shifting automatics a few weeks back.

if you always manually shift your automatic, you may wear out a shifter bushing quicker which will present itself as a slightly inaccurate shifter.

in automatic mode the transmissions shift themselves quite well. Times when you want maximum peformance with a bit more control manual shifting is rewarding. There are some good books that discuss shifting automatic transmissions - point I want to make is to know how professional drivers shift automatics.

No doubt for maximum longevity easy driving with mild acceleration and easy braking will get you the highest mileage.