Aha, now I understand, thanks for clarification. I was confused by the "additional bounces" :-)
I cannot tell you what the others are doing, but in general if you want to simulate light bouncing off the grass, you have to either have that in your sky model (so either the Sky+Sun or an HDRI map) or you have to add a plane for that light (as you were describing). Of course, if you have green grass outside, the reflected light will tint your room green, as it would in reality.
If you use Sky and Sun, you can use the ground color and it will act exactly as if you had a plane of that color outside of the room, exactly as you suspected.
Hope this clears things up