Dont forget about the parallax value. If you dont use it, just set it to 0,0. On start of layout just centered to your screen using scroll to.
After that it should be centered. I`m using Scale Outer and my games currently support fullscreen on Android (Galaxy S3) and Ipad 4.
For using scale outer, you have to make sure that the background is extra big than the window size. My window size is 320x480 but I have to make sure that the background fully covered my Ipad resolution.
Just put important game content in the 320x480 , on the outer maybe put trees , stone or something which dont effect gameplay. Kinda like framing a picture.
So far I , except my first two games I think all my newer game should be fullscreen and no black bar.
I just dont like letterbox scale and the black bars, and I dont understand why Scale Outer is kinda hated by people. :)