Manually blocking known bad exits won't really hurt your anonymity but it's also not really necessary as the Tor Project does that for you.

I was actually objecting to stuff like this: ExitNodes {us}. By interfering with your path selection algorithm in such a strong way, you make it easier for attackers to narrow down your path over time (which gives them an idea of which exit relays to monitor) and intersection attacks also become easier. Then again, you might have good reasons to do exactly that. It depends on your threat model, of course.