Yes, it is allowed! However you are right, if you do it too much and not naturally , you can get a penalty by google. But it is a MUST for a good SEO and generating some traffic. If you do it the right way, you will benefit a lot from social bookmarking and your site will not get hurt. Do it naturally, bookmarking, backlinks and so on, I will not get into details with the SEO right now.
Let's say you start a new website. 10-20 Bookmarks per week is a good way to start.
The reason why some people get penalties is because they OVERDO it. They do like 100+ bookmarks and that triggers the google spam filters.
Great way to get more information about what works and what doesn't is by watching the Matt Curtis videos/google's staff/.