That is a widespread SEO myth. Google employees have denounced it on numerous occasions.
What Google has admitted to looking at is your linking profile. In many spammy situations we may be talking about two of one and a couple of another; but for most Webmasters the IP address profile of backlinks is not an issue.
There is no way to know precisely when and how Google will look at your backlink profile, but if you can judge at a glance that it covers almost as many domains as backlinks, you're probably okay without having to fuss over who is using which IP addresses.