OK,
What happened was that I realised that certain characters, such as ü, have different unicode representations depending if it´s upper or lower case. So that made me think that for each domain with one of these characters it would be advisable to register both the upper and lower case versions.
But then I realised that when you enter an IDN into your browser the first step of the browser is to make it all lower case, called case-folding, and so for this reason registering upper case IDNs is pointless.