Oh, I almost forgot a better way to create posts or pages with no sidebar for just one post/page: Custom page templates. It's one of the strengths of the Montezuma Theme, the ability to create multiple virtual templates. Go to Appearances > Montezuma Options > Main Templates and read the documentation on how Custom Page Templates work. Then create a new Main Template by copying single.php into a new Custom template called no_sidebar.php, for example. Edit the no_sidebar.php file to remove the sidebar code and change the class of content from col8 to col12.
Then for any post where you don't want a sidebar, select no_sidebar as the Virtual Template from the drop-down in the lower right of your post editor.