There is a limitation in Oracle that if u do coalesce once, it may not squeeze through whole of the tablespace, u have to more than once for it to squeeze.
If tablespace is large, it might take long time and high CPU usage to coalesce.
Now which tablespace u want to coalesce,table or index.
Table: As said before in thread, design tablespace and table locations accordingly and put uniform extent size for all tables on a particular tablespace.

Or put pctincrease 1% at tablespace level for smon to work and during table creation put pctincrease as 0% or as reqd.

Test locally managed tablespace before use, there are few bugs in 8.1.6.