Any rule? Delivered as 16x9 or 4X3 final file?

[BrianV]

Note topic, but I know the rule was that by hook or by crook, the final aspect ratio is 16X9. However, should the final delivered size be squeezed 16X9 or can it be letterboxed into a 4X3 frame.

I ask this, because we shot letterbox. We can crop if need be. The question is... need we... be... so. Yeah.

So what then? :)