From Appendix 4 of 91-RVSM:

An automatic altitude-control system should be operative and engaged during level cruise, except when circumstances such as the need to retrim the aircraft or turbulence require disengagement. In any event, adherence to cruise altitude should be done by reference to one of the two primary altimeters

For more information on DRVSM than you could ever possibly want, cruise on over to http://www.faa.gov/about/office_org/.../enroute/rvsm/