@VectorStyler It's fine to set that field to mirror a default, but optionally, it could be empty/deactivated until actually needed. Perhaps prefill with the document resolution or a sticky value (ex: "last used"), etc.
A benefit of the field being deactivated without user input is that it would be more user friendly to add additional "hard coded" values such as 96, 144, etc, to the dropdown instead of "Low, Medium, High". This is an aid for people working with web/screen graphics or needing it for other requirements. Since the activation of the field is "not required" to export, then having those extra options available is an added convenience rather than a nuisance since they are both not having to write out a common unlisted value each time and they don't have to use the drop down at all if they've set it in document.
Where odd-ball DPI/PPIs are used (app design, iconography... maybe?...), perhaps then a "last used" sticky value could be remembered in that field, but I personally almost never run into that situation. If I did, I'd just set the document up differently.
The only other thing I might add is if the field is deactivated by default, be sure to print the PPI value in that area so that the user sees what is actually intended for export. So if the user goes to File>Export, it should say the PPI will be whatever the document was set to... and then optionally below that, the field and the drop downs to change that.
I always thought of that area as an optional setting since we've already set the PPI once when setting up the document. There's not many cases I would need to "set it again" unless I were exporting to a different resolution.
I hope that was clear.