Matt Cutts was recently asked that is you have 20 domains is it okay to link them all together? To answer this questioned he posed another question, “Why do you have 20 domains to begin with?” It does look very strange for someone to have 20 domains and a need to link them all. Cutts went on to say that there are a few instances where you might have a large number of domains where it makes sense to link together. Say you had similar sites but for different countries or say travel sites for different states. It may make sense to have these linked together as one might have the need to go from one website to the next. Cutts did note that having these all linked in the footer is not the best idea. He suggested that having a drop down menu for the pages or even a single page with HTML links used for navigation would be preferable to having all the links in the footer.
I think the most important thing to draw from Cutts suggestions is content relevance. Are the sites that you are linking to one another of similar content? If the content on the sites is drastically different, then odds are you are simply spinning a web of links for SEO purposes. This type of grey hat SEO is spammy and can lead penalties from Google.
Hear what Cutts had to say: