I think that having sites on different c-classes may help a little, but there are other things like what the name servers are and domain registration information that can indicate that sites are all hosted together but the same owner.
What we did was get some different shared hosting accounts and now have about 8 of them with different providers. We try to distribute sites on the same topic to different hosts and then link many of them together. We also have about 60 blogs the same way and use them to get links here and there to different sites on the same and different hosts.
From what I have seen, it is more important to have unique content and not try too many "tricks", rather than spend much time and energy on things like SEO hosting. If you have some "good quality" sites, you can host them all on the same IP and interlink them and they should do fine. But you have poor quality content, duplicate content, the same templates, etc. then I think using c-classes, different WHOIS information, and other things are not going to help as much as you might like.
Agreed! I think in the end what really matters is how unique and meaningful your content is. Furthermore, if your content is providing any value to the users or not... And, yes, a good , stable and reliable web host is a must, and cannot be compromised. All of your hard work would go futile if your sites cannot be accessed. So I would pay more attention on creating quality content, and getting good back links...
I agree with nielsencl. Nice advice as your experience.
Quality content is the most important thing. But, I do believe that hosting domains on different servers with different ips helps. Atleast, if you are link your one site from other, you will give advantage to it. I use less expensive seo hosting services from www.dinsol.com. They do not offer multiple c-class dedicated ip but they offer choice of three data center which really helps.