Alphabetic Order in Category Lists

[rpatters1]

Or in a traditional library, "For tea and tea, two (2)"

[lurchi]

As we talk about alphabetic order...
Is there a rule about the alphabetical treatment of german umlauts? At the moment they are not treated consistently; if you look e.g. for a composer Müller, you find some of them after Mulholland and some after Mysliveček. Some contributor might enforce (by DEFAULTSORT) as a third sorting option the german sort order (ü=ue etc.).
Thanks for the enlightenment -
Ulrich