I am not a trainer, but following the Airtours runway excursion made a firm decision in my mind to deploy the speedbrake before using the reverser.
I understood it to be that the reverse thrust dumped a load of lift off the wing and if reverse was applied on one side without any speedbrakes (spoilers for the technocrats) then the weight would be unevenly loaded and cause a harsh swing - but I like the rudder lift theory mentioned above and it does help explain something else I have seen recently.