An addition to your very comprehensive and useful tags post- In order for a post to show up in tags on tumblr search (the little search bar in the top right corner of the dashboard), the tag has to be one of the first five used. So a post with the tags #happy #green #wow #yellow #trees #photography would NOT show up in the #photography tag in tumblr search (although it would show up if you searched for that tag on your blog).
I know that used to be true. Do you know if it still is? I know some things about how tags work changed recently.