Hi Meg,
first of all always consider this:
How to use categories in your profiles
Always use the lowest possible category in your ancestor profiles, usually the town/village and don't put them in North Rhine-Westphalia or other higher level categories. Please use search to find it first: https://cse.google.com/cse/publicurl?cx=003048598688029858478:aemrovxydzo) and if it's not found enter it in its German writing with a comma and the State it's in.
Eg: Köln, Nordrhein-Westfalen and not Cologne, North Rhine-Westphalia
-------------------------------
Also, that's why I wanted to have a link to this guideline on every location (as part of a link or a template, however its technically done), please read the guideline carefully, it hopefully answers all your questions (otherwise we should add the answer to your questions):
http://www.wikitree.com/index.php?title=Space:Structure_and_how_to_use_regional_categories_for_Germany&public=1
------------------------------
If you search you will find out that both Preussen as well as the Rheinprovinz already exist as categories and both are properly assigned:
http://www.wikitree.com/wiki/Category:Rheinprovinz%2C_Preu%C3%9Fen
------------------------------
You will also find out that Bonn is actually wrongly assigned (where Vilich should point to), it must be Bonn (Kreis). Will correct that in a minute.
There is absolutely no problem with assigning historical locations, it's actually welcome. It's just that we don't have the time and even correcting all those location categories that are constantly entered wrongly in Germany aren't properly taken care of.