Look at the sites one at a time, and devise a linking policy for that site, based on related links that will be useful for visitors. SEs (especially Google), aim to emulate the visitors POV, and non-related links may lead to downgrading.

There's also the incest test - getting too close to siblings can lead to being ostracised from the community.

I use my other sites to help launch new sites - I'd be crazy not to, and I'd do the same kind of thing in the 'real' world - but once a site is up and running, I remove all but closely related links from my other sites. Much healthier for a site to stand on its own three feet.

For sites that use even possibly dubious methods, such precautions would be vital - otherwise if one site gets MRSA*, then all may be infected. And if it's streptococcus**, you could lose an arm and a leg. But keep all these things in perspective, or you may end up with a psychosomatic disorder***.

*The Internet equivalent is a bad neighborhood
**The Internet equivalent is a Google Ban
***The Internet equivalent is a 950 Penalty