I mostly (rather I would say always) play at "half" level. It gives me the 3ds effect without hurting my eyes and effecting the battery life.
And for those who see BLURRY SCREEN when turning on 3d feature on your 3ds, do this:-
1. Keep your 3ds in front of your face (at least 14 m away)
2. Always look from the front level of your 3ds.
The basic conception of the 3d feature in the 3ds is that it gave out two images. One for your left eye and one for your right eye. When you see both the images at the correct position, both the images will be produced on your eyes and before reaching the brain, they somewhat combine so as to give a 3d image. So, in my opinion you should use your 3d feature at half level. It produces 3d images without hurting your eyes. Hope you found this helpful. Thanks.