I can give you three answers for that question.
First, you are not required to allow readers to comment on your blog. You control your level of comment acceptance and it can range from allowing no comments to allowing anyone to comment “live” without prior approval. The wisest corporate blogging strategy falls somewhere in between. You can keep comments totally turned off for some entries, and open but monitored on others. You can pre-register your commenters to allow in the people you’ve heard from before. You let them in because you trust them.
Second, you will not get so many comments—good or bad—that they overwhelm you. Even the best and brightest corporate blogs have pretty small comment numbers. Companies should use blogs to tell their story differently. If you do that, you'll be attracting a friendly audience, developing trust with your readers and they, in turn, would likely not be inclined to leave negative comments. At Stonyfield Farm, for example, our most common type of blog comment is something along the lines of “Great work! Keep it up.” If you get 10 comments on one topic, that’s a landslide.
Third, you may want negative comments because they will further your own communications goals. It sounds improbable, but consider this example. Let’s say you want everyone to know that your company is 100 percent AGAINST the senseless slaughter of hamsters so that their furry little coats can be turned into cell phone holders. You want to say this loud and clear in your blog. Then, you want the other side, the Anti-Hamster Faction, to be able to have their say. Why? Because it will make them look like jerks. This is not rocket science, but it is an age-old trick journalists use. You let the source prove the point for you.
In short, your corporate blog is like your house. You make the rules here and you’re allowed to kick anybody out who doesn’t obey them. When to allow comments should be part of your discussion about starting a blog, but you shouldn’t let fear of comments rule your decision.