I'm questioning the value of bookmarks to a link building campaign? I've run some experiments with booking/backlink from PR2 blog versus pure bookmarks. The pages with the "pure bookmarks" still have not been cached while the pages with bookmarks/backlink are appearing in the organic results.
Here are some benefits of social bookmarking:
it provides natural links
It helps generate traffic
I would bookmark every article. It depends on how much time you have for bookmarking but there are tools to speed up the process.
The traffic that you get from the bookmarking of sites is minimal. Thus, you really don't get enough return on investment to justify the bookmarking unless you get to the front page of digg or something. So, the question is whether the SEO benefits provide enough benefit to cover the time and expense of bookmarking your sites.
The SEO implications are twofold. First, the bookmarks themselves appear in the SERPS where you achieve "optimized promotion". Using longtail and less competitive keywords can increase your traffic and give you keyword dominance. The disadvantage is that bookmarks are generally shortlived and disappear after a few weeks. The second and more important implication is the deep backlinks that social bookmarking provides to a website. In the past, I've done quite a lot of bookmarking and have mixed thoughts on the subject. Though lately I noticed that the pages bookmarked have been indexed faster and deep pages are appearing in the organic serps. The jury is still out on it in my opinion, but there are enough positive benefits. So that you should do it as much as possible.