Bud shared part of a post from Tim about lies we tell our students. Bud mentions the lie of the 3-5 sentence paragraph.
We talk a lot in our Writing Project institutes about not giving students these formulaic views of writing. As readers we know we don't enjoy reading things that follow these "rules" and yet many teachers tell their students that they are RULES that must be followed.
Anyway, check out Bud's post and Tim's if you haven't seen them yet.