How to tag lines which are part of a boundary?

Should lines which are member of a relation type=boundary have any tags? Should they have “boundary=administrative”?

Possibile solutions:
a) The line should have no tags.
b) The line should have only boundary=administrative but without admin_level - but in this case Osmose throws an issue (boundary=administrative without admin_level).
c) The line should have boundary=administrative and admin_level - but which level if a line is part of a boundary of different levels: country, county, city…

Which tagging is correct?

The official answer is a) because the attributes of the line can be derived from its membership of the boundary relation. This seems to be good enough for the standard renderer to do the right thing. But I believe adding the extra tags is superfluous, but not actually wrong. It’s similar to access=yes or oneway=no. I hate it when people share the boundary way with tags for waterway, highway etc - it’s a maintenance nightmare. The boundary ways should exist in their own right, and therefore some top-level tag would seem appropriate to me.

I personally prefer c) because my editor of choice (Potlatch2) styles the ways only according to the tags on the way, which leads to a spaghetti of narrow black lines. If the boundary ways at least have boundary=administrative on them then they are distinguishable during editing.

This may be a problem for other people as well - I frequently see cases where footpaths (for example) and boundaries have obviously been confused during editing.

So it won’t be a problem if I remove the tag boundary=administrative from such lines?
I am fixing Osmose issues and didn’t know what to do with these ones.