Gary
agree about spreading them out over time. the secret is each of us has an Ip address, this is what makes it real to google and not spam. this is how google tells if an account is fake. you would not hold two or more account at one bookmarking site with the same IP
so from google point of view lots of social bookmarks all with different ip's that's social verification that your site is real and that real customers are bookmarking it. its the same as a customer writing a comment on line.
Respect
Ian Harper