Alright, I'm trying to get discussion moving along with other mods but in the mean time, I'll offer my two cents.
First things first, generally it is best to link to the actual creator's page as opposed to boorus. The biggest exceptions being if the actual source is NC (ie the image uploaded is part of a photoset on say Tumblr, and the image uploaded is fine, but all the others are basically just porn). There it is better to link to a SFW booru or just say the name of the creator and what site ("so and so" on Tumblr).
Now, one thing of particular note where I personally think a NSFW Booru is fine to link to (as long as the immediate page on the link is not NC) is when the original artist has taken down their work. Documentation on a booru is better than no documentation. Generally, though, you still want to put in the fact that this link is going to a NSFW website, even if the first click is fine.
Tumblr blogs get iffy, for multiple reasons. A primarily porn artist/ porn reblogger may make a few SFW pics that you want to upload. Generally I would go with the same thing I said about the tumblr photoset, say who it is and what site, but do not link. Of course, it's possible that a mostly SFW artist decides to make said blog porn related, and have porn in their theme or something at some point in the future. There's not really a lot people can do about that.
Pixiv and DeviantArt generally requires you to login to see NC things. There are people who don't know how to tag explicit stuff correctly, but overall, if the image doesn't break the site rules itself, then it's fine to link to on these sites.