You don't need an IIF() at all here. The comparisons return true or false anyway.
Also, since this row visibility is on a group row, make sure you use the same aggregate function on the fields as you use in the fields in the row. So if your group row shows sums, then you'd put this in the Hidden property.
=Sum(Fields!OpeningStock.Value) = 0 And
Sum(Fields!GrossDispatched.Value) = 0 And
Sum(Fields!TransferOutToMW.Value) = 0 And
Sum(Fields!TransferOutToDW.Value) = 0 And
Sum(Fields!TransferOutToOW.Value) = 0 And
Sum(Fields!NetDispatched.Value) = 0 And
Sum(Fields!QtySold.Value) = 0 And
Sum(Fields!StockAdjustment.Value) = 0 And
Sum(Fields!ClosingStock.Value) = 0
But with the above version, if one record has value 1 and one has value -1 and all others are zero then sum is also zero and the row could be hidden. If that's not what you want you could write a more complex expression:
=Sum(
IIF(
Fields!OpeningStock.Value=0 AND
Fields!GrossDispatched.Value=0 AND
Fields!TransferOutToMW.Value=0 AND
Fields!TransferOutToDW.Value=0 AND
Fields!TransferOutToOW.Value=0 AND
Fields!NetDispatched.Value=0 AND
Fields!QtySold.Value=0 AND
Fields!StockAdjustment.Value=0 AND
Fields!ClosingStock.Value=0,
0,
1
)
) = 0
This is essentially a fancy way of counting the number of rows in which any field is not zero. If every field is zero for every row in the group then the expression returns true and the row is hidden.