Hello David, no, that is not necessary. As these 6 1/2 DMMs (34401A architecture) have a LM399 reference, there is no (known) storage effect. Also, it's a 6 1/2 digit or 20ppm class only, therefore any such effects will be deeply buried in the uncertainty figures and noise. The 3458A and its LTZ1000A might suffer from long storage time, but this was reported only for a certain range of S/N, and you'd only encounter an effect if watching out for highest stability, in the sub-ppm area. If you'd only look on the standard specification area, i.e. ~ 2ppm, you also would not see any effect. Frank