Ishuli and Rahsael have said it, but to only kick the dead horse once or twice:

In terms of an RC, I rate based purely on the role that is offered to me. If I enjoy the role, then it gets a good ranking. The roles I feel are worthy of reward, get rewarded.

In terms of late adding: If you have the role in mind, and you RP that role without adding it, I will not fault you. If there is a difference in your role and your RP? Yes, it can hurt. So, my suggestion is to RP from the beginning based on that role, and add it when you are ready.