Depends what you're trying to achieve
If you want to diversify your link profile, you only need a few types of backlinks (web 2.0s, profile / foundation links, some quality .edu / .gov links, social shares, etc) pointing directly to your site (so, no other tiers)
If you want to rank, those web 2.0s won't do you any good, no matter how many tiers you build to them and what types of links are on tiers 2, 3, 4 etc.. I mean, they could work if you waste powerful links to boost the web 2.0s, but what's the point in doing this when you can point the powerful links to your money site directly?? Just get some quality PBNs, quality guest posts, or quality link insertions (niche edits) and you might not need any other tier. And if you will need tiered links, you can use the web 2.0s as tier 2, and everything else (wikis, article directories, bookmarking sites, whois / stats links, redirects, etc) as tier 3 and that should do it.
define "high quality web 2.0s"! Because I don't think there's a single web 2.0 that's quality. I mean, they look like quality sites, but the SEO value you're getting out of that good looking site is minimal at best
depends on a lot of things:
indexing - depends on google's mood (first), the quality / authority / popularity of the website that gives you the backlink (second), the effort you put into indexing the link (third), your luck (fourth)
ranking boost - depends on your keywords' difficulty and niche of your site (first), the quality of your content and the overall authority of your domain (second), the number and quality of RDs and backlinks your site has (third)
