There is no finite rules for tier 2 and beyond but the general theory is that you can let quality lapse in the later tiers. You can use WordPress, blogspot, squidoo, blog dot com, etc directly to your primary domain. You can then create more WordPress, blogspot, etc and use them as tier 2 without any issue.
Personally, I don't like using low quality anywhere throughout my tiers because it is my belief that Google will crack down on it eventually. It is a lot more work but I'm in it for the long haul. I have been ravaged by Google's critter updates previously so I play it safe and only use poor quality link pyramids with unimportant domains. That being said, you can use blog comments, wiki links and forum profiles in your later tiers today and it works like a charm. Just do not use these as your tier 1. The web 2.0s that you mentioned will do fine as tier 1 to tier 99 as long as some nice high quality unique content is placed on them.