Well, I can tell you that it is a rare to hear an anchorman or woman with one of the nasaly midwest accents. In most cases individuals have a slight accent on larger networks, but they attempt to play them off for the sake of everyone listening to their reports. Many of the field reporters let their accents slip once in a while. Personally, I enjoy hearing the small-town accents because they are sometimes absolutely ridiculous and make me feel at home. Like people saying "beggle" instead of bagel. I would say accents really come out on sports radio. I guess it's a way to drawn-in the regional crowd. Most importantly, a network can not say that someone will not be hired due to their accent because that would be grounds for a discrimination lawsuit (as ridiculous as that is).
Personally, I do not have an accent, but I am good at imitating them because I grew up overseas.