You're probably putting more stress on it by going in and out of gear than just leaving it in gear.
When I get lazy, I'll yank the parking brake and take my foot off the break, but I'll still leave it in D.
Funny, I'm too lazy to keep my foot on the brake, and in less than 2 weeks, I'm trading in for a car with a 6 speed manual (my first manual). At least I can still use the parking brake at stoplights
Oh, and on an auto, going to neutral once stopped might not be too bad (probably more stress and wear, but I doubt enough to significantly reduce the life of the tranny), but NEVER, EVER go into neutral while slowing down to a stop, for 2 reasons:
1. You lose the added benefit of engine braking if the car's not in gear
2. If you have to quickly avoid something by accelerating and moving out of the way, you'll hit the gas and won't go anywhere
3. If the light turns green before you come to a complete stop, you'd have to go back into drive while moving. And you should only go into drive when stopped, really, really bad things may happen to your transmission if you shift to drive while moving.
Manuals are different, as you can go from neutral to gear while moving (just make sure to select the right gear for your speed). Hell, you're basically going from neutral to gear every time you shift since the clutch in is basically the same as neutral. But when stopping in a manual, from what I understand, brake while in gear, and just before the engine gets to stall speed, clutch in and shift to neutral. This is the same as reason 2 above, you want to be in gear for as long as possible for emergency maneuvers.
In short, an automatic transmission is meant to be, well, automatic. Put it in D and leave it there until you need to park (or back up). If you want to play with your shifter more than twice each trip, buy a manual