That may very well be true but it may not be. Many who optimize for search engines not only get on their own IPs but ask for them to be on different blocks. What is true is that search engines penalize for sites that they see as spam. It seems to me that if all of your sites are on a shared IP with the same registrant information it would be quite easy to connect the dots and blacklist sites of little value with duplicative content.
I believe most that play the search engines agree that the obscurity certainly can't hurt, especially if you are publishing dozens, hundreds or thousands of domain names.
We are very generous with IPs!
Brad