you should have clustered index on every table .

that is what they called as best practice.

data is not organized properly in heap tables (tables without clustered index)

http://msdn.microsoft.com/en-us/library/hh213609.aspx

http://technet.microsoft.com/en-us/library/cc917672.aspx

http://www.simple-talk.com/blogs/2007/12/17/should-you-always-use-a-clustered-index/