There are lot of discussions going on rebuilding indexes..
In simple,the main criteria to rebuild an index is
1.It should not have the BLEVEL more than 4 (you can find these BLEVEL of an index from dba_indexes )
2.Deleted entries should not be more than 20%
here is a query to detect the percentage of deteted rows and Blevel of an index.
a.name,a.del_lf_rows*100/decode(a.lf_rows, 0, 1, a.lf_rows) pct_deleted,
distinctiveness, b.blevel blevel
index_stats a,dba_indexes b
where b.index_name = a.name;
Make sure u have analyzed the tables before running the above script.
And also i like to share this link where you can find a healthy discussion about the Index re-organization..
This is an Excellent Discussion going between the DBA gurus..
Be paitent while reading …hope it helps……..