Do you mean the
Settings Nodelet -> User Settings -> Miscellaneous -> No Forced Preview
checkbox? According to Help for User Settings:
No Forced Preview: Checking this option will make it possible to submit a node without first previewing what you’ve written; in other words, without reminding you to check that your post looks the way you intended. This is a bad idea except for those very wise in the ways of Markup in the Monastery and then, only for those who never make typos or other mistakes. As Writeup Formatting Tips notes, “Preview, Preview, Preview.” For the same reason you faithfully
use strict; use warnings;, leave “No forced Preview” UNchecked.
Hope that helps,