PhD course grade impact on postdoc and fellowship applications?
In my field (math), nobody looks at your grades, so this will not make any difference. 

A personal anecdote: I had a first-year grad course with a tough grader who gave me a B- in a core subject. This did not affect my trajectory in the least; I still passed my quals ahead of time, I wrote a thesis and graduated on time, and I'll be starting my post-doc in the fall.