What's the need of sidebar on pages which have no relevance with these pages. Tags, Users, Category pages should be without sidebar so that complete pages can be used to show tags, users and category respectively.
We can also see this from stackoverflow that there is no sidebar in tags page, users page. Use complete page for tags and users.
For categories, show categories in half page and in front of categories, show latest questions asked in that category. Something like show in image below.