Never use it. It's a resource hog. Every site you visit is stored so as to be available to display as a Top Site.
To completely disable Top Sites, go to Safari > Preferences > General
New windows open with: Top Sites
Change it to something else. Obviously, you can do the same for New Tabs.
(You'll also see Top Sites in the Show All Bookmarks window. Drag that up, don't stop when it first stops moving. It will be reduced to a thin bar.)
Now delete the Image Cache. Choose Safari > Reset Safari... and select Remove All Webpage Preview Images. Click Reset. Now go to YourUserName/Library/Caches/com.apple.Safari and select the Webpage Previews folder. Press Cmd-I and click Locked. Now it won't accumulate a jpeg and png of every web page you visit. I saved over 500MB of disk space!