I have just received approval for my IP request. Do I have to inform the RIPE NCC on which IP range I am going to assign to this network ?

No, there is no need to inform the RIPE NCC on the IP range you choose. The RIPE NCC only keeps a record of the approved netname and the size of the approved IP request.

As long as you don't increase the size of the assignment or change the netname the assignment will remain valid. This means if you ever were re-organising your allocation, you can move an approved network to another part of your allocation without having to inform us.