I've seen startups launch without one and fizzled until they reached out.
What kind of details are important? Who do you reach out to? How do you get their attention? What are some dos and don'ts?
I've had some success using this web based service. They get your release linked quickly and provide advice on how to write it. It has to be news, not a sales pitch.
You might find this free tool useful:
http://pressrelease.grader.com Disclaimer: My company (HubSpot) developed the tool. It focuses primarily on some fundamental analysis and SEO related things.
Make it coincide with, or relate to, an event, holiday, show how it solves a problem, make sure to pitch to all of the trade publicrelateions who cover your specialty, along with magazines, radio, newspapers.
Things are flowing into social media now with PR, so you have to have social media strategy on all of the major ones, reserve or buy up your brand's name everywhere (check knowem), blog, have whitepapers, have a newsletter sign-up. Some, or all, of this may apply to your industry, but if you are proactive, as the 2.0 medium requires proactivity, it will pay off in the end.
It should go without saying that you never get a second chance to make a first impression, so make sure your messaging, strategy, brand and appearance are all looking good before you market your company/product.
As a journalist the most important thing to me is that someone gets their key message right and sends the press release to the right people! (It is very annoying when I cover small business topics to get a press release about something that has nothing to do with small business.)
We have a free press release builder workshop on our site that can help you with all the technical details.