Decided to go straight to the source so here is what Kodak said:
Film curl is due to humidity contraction of the film when stored in a
wound configuration at a relative humidity of less than 40%. This is
most often a temporary or reversible change, and can be correct by
rehumidification of the film. Allow the film emulsion to reabsorb
moisture while in an atmosphere of higher relative humidity. Use of a
humidifier in a closed room will accomplish this, but be sure that the
mist does not get directed onto the film itself. The film will unroll
to an extent with rehumidification. Once this has occurred*, you may
then place the film into an archival storage envelop or sleeve, place
beneath a weight, and then bring the atmosphere closer to 40-50%
relative humidity to retain its dimensional flatness.
*be sure that the film is not moist to the touch.
So there you have it, not too dry and keep a light pressure on to maintain flatness.
I tried to buy some more of the accordion folding type of negative storage sheets but none of the local photo stores I contacted carry them any more. Seems when closed, they kept just the right amount of pressure to keep the film flat. Has anyone using the binder style of negative storage sheets noticed any tendency of the film to curl after a long time in storage, especially if the relative humidity is low in the house like during our winters?