mariest i think so, at least this is how Oracle whitepaper suggests, since 128K is multiple of 64K and if I am correct I think most OS I/Ois 64K.
Oracle suggests db_block_size * db_multiblock_read_count tooptimize full table scans, if that was the case it would be 64K, now this is my personal view i think 64K extents are simply too small in a considerable size database so i set it to 128K. If we use 64K we will have tons of extents (which i dont think is good) even Oracle states that having many extents doesnt affect perfomance (this was said by an Oracle Senior instructor) I do think it affects since the hard drive head would have to move too much in order to retrieve data.