Links need to come from different Class C IPs to be any good for SEO right? So if you have 100 links from 100 different subdomains its not going to help SEO much b/c they will all be from the same Class C IPs.
As long as they links are one way, and you are not linking back, the most they can do is not benefit you, unless you are linking to bad neighborhoods.
Over half of my backlinks come from other sites on my server with 3 sequential IP addresses. Some of the sites are mine, some are from my friends and my web hosting customers. These links are the backbone of where my pr is coming from, so I don't think it matters as much as some make it out to.
Different ip matters, Class C also matters but change is negligible. Themed link partners is the should be the aim before starting to get link partners.