Reversing a cell can prevent electrical contact at one end of the battery in SOME flashlight designs. This would prevent accidental drain if the switch is accidentally turned on. This can be a good way to protect the batteries.

However, if the cell makes contact and the switch is turned on, it would short out the batteries, which would not be a good thing especially with lithium batteries. It is better to install the batteries correctly, but have a method of locking out the switch.

Lithium batteries will typically have a much longer storage life in or out of a flashlight, and lower chance of leaking. Ni-Cd is not suitable for long storage because they lose a significant amount of their charge each month even if unused.