If you have sites on the same server and link between them, Google probably devalues those links to some extent for ranking purposes. If the sites are on different servers, then the links will probably carry more weight - until Google recognises the sites as related in some other way (whois info, reciprocal linking, members of same link farm, etc)
If you are not going to link from one site to another, then it probably doesn't matter.
Whatever you do, don't have reciprocal links between every site - those links are likely to get ignored for ranking purposes.
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