It really just depends. Are these throw away sites that you don't care about or are they part of a long term link building campaign?
Remember you can build mini authority sites with the web 2.0s. If you treat these like extensions of your own site you can build them up also in search engines and use them as traffic generators as well as strong backlinks.
When I used to focus on web 2.0s I would actually try to rank them also. At one point I had 7 of 10 results on the first page using web 2.0s, videos, and my site in one niche.