I would think that it is in general best to have unrelated breeding stock ...... but someone may come along and run you through the complexities of in-breeding, line - breeding and so on.
If you really want to keep one of your own cockerels I think you would be unlucky to have major problems at least for a few generations. I suppose it depends whether the eggs that you have hatched are already from quite in-bred stock or not. A neighbour of mine just brings in a new cockerel every few years to bring in some "new blood" and her hens always seem to be healthy and robust.
Thinking about it ..... most of the people up here are related in one way or another

and they seem okay.