Arima, that's not entirely true.
Jase00, if you go, use tiled background and set it to be invisible (not transparent, invisible).
I had a big headache after doing a big mask with many polygons, because the engine check the collision on a runtime, and when it get low fps, the problems start to happen with complex polygon areas.
Long, long time ago I reported this bug, and it was solved, but Ashley told me to avoid using big collision masks.
<img src="https://dl.dropboxusercontent.com/u/47035927/temp/collision_issue.png" border="0" />