Alexban.
It may well be a good idea to check with a current Area or Approach controller. As I said, it's a long time since I was at school.
You are exactly right if you are given a specific heading to fly that heading. If you are given a RADAR heading, as I remember it, the controller expects to see your paint parallel with the PPI cursor on that heading. He can't give you safe track separation if you don't correct for drift. Fine if your all on the same compass heading and TAS.
I'll be interested now to know if I have remembered it wrong.