If you are coming from an Oracle environment, the term "reorg" is a big deal. In an Oracle DB, if you have done any archiving, a full reorg of the affected tables would have to be performed in order to utilize the space freed up by archiving. This is not so in SQLServer 2000 and above (and I believe SQL 7 as well). SQLServer will automatically utilize the free space in a datafile at the block level so that reorgs are not necessary to regain free space.
Now, if you are performing a reorg in order to increase performance, you should consider ordering your data in such a way that SQLServer spreads the data across all datafiles evenly. All datafiles would then have a somewhat equal portion of High and Low I\O rows, thus optimizing overall performance.
It is possible to manage performance at the table level and assigning tables to specific datafiles that you have placed on a faster array, but this level of granularity is likely more work than it is worth.
