As I've stated before, it is pretty easy to draw true line of sight from a cannon through intervening models such as skinks. I agree with
@Karnus that you must be able to draw line of sight to the point at which you are aiming (whether that is a model or a spot on the ground).
So it simply comes down to this:
- take a laser pointer and place it at the muzzle of the cannon
- if you can make the laser hit the spot you which to aim at, you have fulfilled the requirements of true line of sight
My point is that it is easy to aim that laser over (cannons are typically placed on hills), between models or through various parts (between legs, under an arm, over a shoulder, etc.) of a small unit of skinks (don't even get me started if they are skirmishers). If the unit is of sufficient size or the fates align perfectly it is possible, but you only need a millimetre or two of a gap to get that laser pointer through. With a unit of Chaos Warriors, who are very bulky models and are tightly condensed, it is a reasonable tactic. However, for a weedy little unit like Skinks, I don't view it as a viable, reliable and consistently repeatable tactic.