In version 7, the indexes will be reorged along with the table. In version 8, there is a separate command to reorg indexes.
When you chose an index to reorg the tables with, DB2 uses the index columns to order the table rows. If you PK has only one column, then that is "probably" the wrong sequence to order the table rows (assuming that you have other indexes on the table).
Generally speaking, one index should be defined as the clustering index, so that inserts will maintain (if possible) the correct sequence of the data, in addition to when you do a reorg of the table.