Hi Lasse,
The changes to these users will affect not only during the installation but also on going support. This is due to the priviledge of this users to SAP systems. see more detail in the below link
http://help.sap.com/saphelp_nwpi711/helpdata/en/0a/0a2e51ef6211d3a6510000e835363f/content.htm
I would recommend to advise customer to follow SAP standard.
Regards,
Casmito