If reasonable number of press releases has been submitted manually, they work quite well for driving traffic as well as for facilitating online presence of a site in question. However, press releases are unlikely to result in a massive backlink boost, and that's why any penalty is unlikely to follow-up.
The power of press releases - apart from backlinks they provide - is that they actually make possible to share your content with a wider audience and subsequently promote your site. For instance, press releases can actually help to reach journalists, bloggers or editors. This may result in new networking opportunities to be used for guest posts, press articles, affiliate offers. Moreover, press release submissions may get your article right on Google News and, if this has been the case, your site's online exposure will increase and your web pages will get backlinks not only from the PR itself, but from industry news as well.
Even though you distribute only a few press releases - they will still be beneficial in terms of rankings too, especially if you target low competition keywords. So, if you use press releases not entirely for getting backlinks, but for sharing your content and reaching customers and niche players, this practice is absolutely legit and won't result in getting your site penalized.