If you're using press releases for backlinks you've missed the point.
The opportunity in press isn't a short-term seo benefit... the articles from a release blast come up and go away faster than search engines care. PR is about exposure to millions of people driving traffic, awareness and hopefully positive hype. There is far more to marketing that just SEO.
Good results from PR comes from stories covering your site. You don't get those by blasting out a release automatically or even by sending it to sites by hand... You get it by developing relationships with relevant outlets and pitching them something that they can use to create a relevant story. That gets augmented with a wire release which other outlets are more likely to pick up because the see the lead stories created. Good coverage, relevant exposure and maybe a few links here and there but that's just a side benefit.
If you're still convinced it's all about the links just remember that quality counts and a thousand duplicate articles on a thousand sites that publish nothing but duplicate articles isn't worth much of anything.