i've never had issues with an entity being visible but not having it's clipped flag set
I can imagine that, because that situation is impossible. The flag gets set as as soon as the entity is outside the view frustrum and is hence not visible. The possible danger here is, that the CLIPPED flag is not set but the entity is still not visible.
Clipped set -> Guaranteed to be invisible
Clipped not set -> May be visible, but not necessarily.
