I doubt Google will penalize you for using the same analytics account, of course you should have a different tracking number for each site in the account though so you can track each one separately.
If they are all in the same account then Google can tell that the sites are related so if you link between the sites, then they will be able to tell the sites are related or possibly owned by the same person/entity. Would Google give less value to links between the sites because they are owned by the same people, I am not sure? Some people will say it is possible since they may consider these links as not being freely given and manipulated, but no one knows for sure.
I have personally signed up multiple related sites under the same analytics account without any discernible issues. If you want to decrease the amount of ties between related sites you manage then I would sign up under different analytics accounts. Google may still be able to determine that the same owner manages both sites though (if you login to the different google analytics accounts from the same computer, same ip address, then they could find a tie between them if they wanted to).