Stephen Kenwright wrote a nice post on this in November, "Should You Disavow Links From Press Releases?
You absolutely should be adding press release sites to your disavow file. Google stated in 2012 that links from press releases won’t help your rankings, so there should be no danger in disavowing old press releases." From: http://searchenginewatch.com/sew/how-to/2383492/press-releases-are-not-an-seo-strategy
The entire post is good and will provide you guidance on working with the press in the future. For your specific case though if a bunch of press release sites were used in a manipulative link manner, feel free to disavow. Cheers!