What I did for KidSpell is put the project in 640,360 fullscreen scale and made it so that the background image goes beyond the boundaries of the layout's size.
As far as I've been able to see across several devices, it worked and covered all the screen every time.
<img src="https://dl.dropboxusercontent.com/u/36472942/construct/forumhelp/KidSpell_Resolution.PNG" border="0">
Being a professional is also knowing when to say no to the client and teaching/guiding him when he's making a bad call.
Stretch the graphics ? In my gamer's opinion it's a very bad call.
Black stripes on the side ? In my game designer's opinion it fits with the black and white backgrounds and should not affect the experience.