That sounds rather ostentatious but this is really more of a "best practice" when playing miniatures games. It appears in some of the books but it is worth discussing here:
The Weasel rule of miniatures placement:
When positioning miniatures on the corner of buildings or at the edge of terrain features, always clarify verbally to the other player if the figure is intended to be AT the corner (and thus able to fire and be fired upon but counting as being in cover) or BEHIND the corner (and thus out of sight unless outflanked).
A simple act like that can save no end of confusion and irritation when you realize a turn later that you did not have the same impression of the state of the game and battlefield.
The Weasel rule of shot decision:
If you are unsure if a figure is visible or out of sight, assume it can be seen.
If you are unsure whether a figure is in cover or in the open, assume it is in cover.
These two stances are intended for use only in cases where it really is quite marginal. Being permissive in both cases tends to move the game along and tend to balance out.