Mine are open all the time unless A/C is on in the summer. Towing, stored, heat on (cracked but open). Moving air is your best friend to preventing mold and other potential problems IMHO.
With the constant warming up and cooling of the closed unit you will get condensation on the walls.
2014 375 BHFS Eagle Premier
2014 Ram 3500 Longhorn DRW CC
6.7 CTD, Aisin, 4.10's