Steel vs. Aluminum
I don't know whether or not this is something that you are supposed to do, but I am not sure that it would make sense to have to. Sure, materials shrink with a drop in temperature. However, the coefficient of thermal expansion for aluminum (mast) is larger that that of steel (rigging). So, if the temps drop really far, the mast will shrink more relative to the rigging, and it would tend to loosen the rigging for you - loosening would only seem to compound the loosening problem. I am not sure if there is another reason to loosen the rigging, but cold temperature really don't make sense (unless I am missing something).