Hi Ankur
I have checked in OKEP and find it only a technical requirement to run SAP program and there is no impact anywhere in FI/CO etc...As I understand that basic rule is whenever you have CO object as accounting assignment system will ask a CE cat 1/11 ..in this case also the same rule is applied but to bypass you have to assign default CE cat 1/11 in OKEP to run the program..
So, don't worry use any CE cat 1 for vendor and cat 11 for customer...
Kamal