It's listed under Transaction Variables, which means it should appear in all transactions, and it's not described as optional. However the PayPal documentation and the IPN simulator are known to disagree in other respects, for example date formats. You should treat it as optional for the purposes of testing.
But don't do too much testing with the simulator. It's really only good for establishing that your verified callback works in my opinion. You ned to do real testing and your research into what fields are present etc using sandbox accounts.