This is a topic that comes up a lot around here. The general answer is that it doesn't matter for most sites, but in some few cases it depends on how MANY other domains share your IP and if some of them are extremely black hat.
IP does not matter. Only reason some sites get a down hand because they are not properly configured on the server as virtual sites. Its a problem of your hosting provider. Virtual servers need special configuration else they look like subdomains.
you would then ask me how to know if my server is properly configured. Err I am sorry I dont know..