Rule of thumb: AUTOALLOCATE or UNIFORM size for tablespaces
I agree with Tom Kyte's 2005+ assessment of this:


  I actually am a fan of system allocated extents now.  It grows the
  extent allocation size as the table grows.





  Autoallocate is not going to cause widespread fragmentation.





  IF you know PRECISELY how big the object is, will be or will grow by -- go 
  ahead, do the math, feel  free to use uniform.  Else, use autoallocate...