I always create as follows

create tablespace XYZ
datafile '/u01/prod/XYZ.DBF' XXMB autoextend on next 1028K maxsize YYMB
minimum extent 128K
default storage(
initial 128K
next 128K
minextents 1
maxextents 4096
pctincrease 0
)

minimum extent will ensure all extents size are multiple of 128K because if I dont set minimum extent even if I set initial 128K extent size will be multiples of data_block_size * 5 and because 128K is normally a good size to optimize I/O.
pctincrease 0 because when I have extents with same multiple sizes (128, 256, 384 etc) I dont have to worry about fragmentation I dont have to coalesce the extents neither because extents with same multiple size ensures that when they are deallocated they can be reused by other segments.

I dont deallocate the extents, Oracle will do itand as long as I keep the extent size uniform everything should go smooth.

You find number of extents of segments in view DBA_SEGMENTS column extents.