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I just want to back up Mark Ginsberg's suggestion to you.  I currently manage several WP sites and I would not do without Yoast on any of the.  Not saying there is not other good SEO plugins available, but Yoast is straight forward, yet powerful.  WP without an SEO plugin can be a mess, and limiting what is allowed to index will certainly help you by leaps and bounds.

Good morning David,

Up front, I'm probably not qualified to give you a best answer, but I can tell you my experience with this topic.

I've used a combination of PRWeb and a couple of free press release submission sites in the past.  What I found with PRWeb is that, quite honestly, my content wasn't really newsworthy.  I knew this going into it.  I'm promoting a brand/product, and really, few true news outlets are going to pick the majority of my releases up.

I did have one good experience with this article (http://www.prweb.com/releases/log_homes/log_cabins/prweb4428874.htm).  My company partnered with a band to do some cross-promotion.  The band is fairly well known in their niche genre, and suddenly this article was linked to and copied by a number of blogs, and news outlets that covered the pairing.  At the time, I knew even less about SEO than I do know, so I didn't really do any hard analysis.  Looking back with the knowledge I've gained here, I know that particular article gave our SEO a good boost, but I can't quantify it.

In the end, I couldn't justify the ROI.  This one article was my success, but the others I believe I could have used in other publishing methods (blogs, white hat article submissions, free press release sites, etc) and received similar results.

However, if you are creating articles that are truly newsworthy, I think it's a great service.  I was really pleased with PRWeb overall, I just wasn't publishing the quality level of content to make best use of SEO and see a good return on my investment.