Personally, I say no. It is pretty rare for an answer to be intended to be an "official" answer, since stackoverflow answers are community answers and are publicly editable. That is really the only justification I could think of for doing this. More common is to just link to a more official source of information.
In terms of someone having access to multiple accounts and voting themselves up, the system has its own protection against that; I don't think that issue is really relevant to whether a company should have its own user.
Further, I it would definitely not be appropriate unless the user was intended as an official representative of the company...and even in that case attribution to the specific person, via real name or nickname, is still better.
Having your username be a company name strikes me as being a form of advertising, which is really not the right place to do it. And in general, it is considered poor form to create an account specifically to talk about your company's product, unless you're using that account to answer questions specifically about your company's product.