At the microlevel (where we pass actual data objects between functions), the difference in the amount of work required between designing data layout "on paper" and "in code" is often negligible and not in favor of "paper", because some important interactions can sneak out of sight.

I do data flow diagrams a lot (to understand the domain, figure out dependencies, and draw rough component and procedure boundaries) but leave the details of data formats and APIs to exploratory coding. It still makes me change the diagrams, because I've missed something.