Face recognition on on tilted images
Tilting your head a few degrees to the side doesn't cause Viola-Jones face detectors to fail.  They are more robust than that.  I suggest trying some experiments to see for yourself.



As an aside: It's not always possible to rotate an image to the extent one might like.  You have a 2D representation of a 3D object.  As an extreme case, imagine trying to take a photograph and simulate rotating the face by 180 degrees around the vertical axis.  You can't, because that would require knowing what the back of their head looks like.  Even tilting around a horizontal axis has limits; imagine trying to tilt by 180 degrees -- which would require simulating the effect of your hair pointing in a different direction.  So while it may be possible to simulate rotation to a limited extent, but there are limits to what kind of synthetic rotation is possible.