Reclaiming space from data files in Oracle
If the data files are growing then any file space you reclaim will be reused as the files grow.  

Reasons that cause me to think about re sizing are:


after moving a table/partition/index from one table space to another
after truncating a large table (example: a logging table)


Reasons not to re size:


data files have to be empty to re size below the high water mark, 11g offers online reorganization where the files do not have to be empty
from here: a change that demands some preparation:


take a backup 
Export the schema objects that are stored in the specific tablespace/datafile you want to resize.
Drop the exported objects.
Resize the datafiles.
Import the objects.
Perform any required maintenance, like grants, recompilation, rebuild indexes

requires downtime
for a growing table the space will be used again anyway