Warhammer 40K Tabletop VII: Common Sense is not RAW.

[banjo1985]

With this being the norm you can not convert your models in any way shape or form which would change their size. A model on a rock (for scenic purposes) is illegal because it can see over a wall a normal model can't. A model on his knees (for scenic purposes) is also illegal becuase he can hide behind smaller walls.

It's at a point like this when the slightly clunky War Machine method of LOS starts to sound good. The size of the base determine how high and wide the model on it is for LOS purposes. Though to be honest, a bit of common sense shown on the behalf of both players in a game makes the point moot.

By the way, I'm by no means arguing that the rule doesn't exist in store and tournament play. I'm just saying IMO it's a little foolish and broken by some of it's own moulds for it's models.

EDIT - I'm fully aware of the irony of expecting some 40k players to use common sense to prevent arguments...but still, eternal optimism and all that.