avoiding penalty

These are signs probably used by google (I have not tested them to be honest) to identify artificially built networks:
Same content verbatim
- Same cookie structure
- Javascript function names
- Linked CSS and JS files
- CSS class names
- Same contact information posted on websites
- Common name servers
- Same/similar images and/or graphics theme
- Site hosted on same IP/block
- Whois information matching
- Alexa contact information matching
- Interlinking of domains
- Common backlinks (indirect crosslinking)
- Same credit card used for anything
- Login from same IP to separate accounts
- Residual cookies from past logins
- Similar file names or linking/directory structures
- Code Comments

Quite a checklist. So, even if you are genuinely building a few websites for your clients, hosting them on the same reseller account, using the same page structure, etc ... and think that you may take the benefit of interlinking them together .... not a good idea. You actually take the risk to achieve the opposite of what you want, and get them all penalised.