Hi Martin,
No, there is no rule saying that a package cannot include anything
that's unnecessary for sid and/or would benefit downstream
distributions. There's nothing that compels maintainers to do so
either; it's entirely up to the maintainer. Personally, for my own
packages I do my best to eliminate/minimize the diff in my packages
for stable backports and Ubuntu/downstream distros in general, even at
the cost of introducing extra complexity to that package. Again, it's
up to you as maintainer to decide whether you would like to deal with
this extra workload, or push it to downstream maintainers/backporters.
Regards,
Vincent