Obviously, it's not a good idea to send out a press release to announce bad news. If your product launch is delayed by four weeks because of manufacturing problems, don't send out a press release detailing these mistakes. Send one announcement once the product is ready. If the press is concerned about the delay, don't worry, they'll ask you.
The only exception to this rule is damage control once the cat is out of the bag. In some cases you may want to issue a release to put the bad news in perspective and tell your company's side of the story.
You should also avoid sending out a press release every week. After receiving "news" on your company every week for 10 weeks, reporters will definitely know your company's name — and quickly learn to trash whatever you send them.