"DC" direct current... Power is supposed to flow in one direction...
In the case of simple electronics (motors), the device would simple run backwards... In complexe electrical devices, you could potentially damage the device. Most are equipped with diodes to stop reverse flow, but do you really want to test it???
Switching one battery around in a 2 battery situation should stop flow... turning a single battery around in a multiple battery situation will NOT stop flow, but will provided less power and could potentially cause damage to the batteries (I know this one from experience, I had a battery melt when I had one of 8 AA reversed - ruined the battery pack)...
All in all, it not a good idea... Honestly, you shouldn't store batteries in your device, even if done so "correctly"... Batteries can explode or leak and can easily ruin electronics... I have/had a brand new batter operated air-matress pump (for camping) that made it one day with brand new duracells... One duracell leaked and ruined the contact points in the pump... POS Chinese made crapola, but still...
Terry