Are do-follow backlinks from high PR websites safe ?

Out of - Facebook, Youtube, Google Plus, Pinterest, Filckr, Imgur and Wikipedia; only Google Plus gives you a do-follow backlink. All other sites automatically add no-follow tag to all the external backlinks. So, it is 100% safe to get backlinks from these sites. Technically, neither they can increase you ranking nor they can decrease you ranking.

Except Wikipedia, all of these are social media sites. Therefore, backlinks from these site may positively impact your ranking.

A nofollow link from Wikipedia is better than thousand low quality dofollow links. So, try to get a backlink from Wikipedia.

Of course. If you want to rank your website on search engine then you will need to get good quality back-links from high priority websites.

You can directly contact to owner of these websites and ask them for approval of guest posting on their websites. While posting on those websites you are allowed to add anchor text to your website and get better ranking as well.