As I understand it, the speedbrakes should be up before reverse thrust is initiated because asymmetric reverse thrust with speedbrakes down gives asymmetric lift dump. Asymmetric lift dump increases both drag and brake effectiveness on the affected side which may cause severe directional control problems. Regaining directional control may only be possible by disarming autobrakes and applying differential braking, a course of action especially undesirable following a high speed reject.
Asymmetric lift dump is, to the best of my knowledge, what caused the Airtours 757 previously mentioned to run off the side of the runway (although this incident was after landing following hydraulic failure rather than a rejected takeoff).
In certain Boeing aircraft, with the speedbrake down (i.e. not armed), raising either reverse thrust lever to the interlock stop mechanically moves the speedbrake into the armed position and it will then deploy if the autospeedbrake system is operative and the relevant conditions are satisified (which they should be following an RTO).
However, in the light of lessons learned from the Airtours incident, following an RTO or after landing with certain hydraulic failures (when asymmetric reverse thrust is especially likely and autospeedbrake may be inoperative), it is now considered preferable to manually raise the speedbrakes before selecting reverse thrust and not rely at all on the auto system.