The only situation I can think of where there could be a change from PHPExcel 1.7.0 to 1.7.1 is in a situation like this.
Consider this line:
In PHPExcel 1.7.0 you will probably see a blank cell when you open the generated Excel file, but in 1.7.1 you will see a 0.
What is happening here? When you use PHPExcel_Cell_DataType::TYPE_NUMERIC, the first argument is really expected to be a number. But
'' is not a number, it is an empty string.
In PHPExcel 1.7.0 such code could result in corrupt Excel files (xlsx). Try this with PHPExcel 1.7.0 and you will see that you get a corrupt xlsx file:
In PHPExcel 1.7.1 a mechanism was added to protect against such errors
That could explain why you get different results, but the above should explain why the approach of PHPExcel 1.7.1 is the correct.
To summarize, you should only use PHPExcel_Cell_DataType::TYPE_NUMERIC when you really want a number. A number cannot be blank. If you for some reason want a blank cell, use an empty string:
Note: Do not confuse data types and number formats. The above has to do with data types and not number formats.