In the worst case, a cell of 10% or lower capacity among a string of other cells and running down to complete darkness, cell reversal is a bad thing. In the more likely case that you have a set of cells that vary over no more than a 5% range (that's 100mAh on a 2000mAh cell), and you smartly turn off the light once you see it begin to visibly dim, it is likely never to become a problem. The first case is partially abusive use and that is under your control.
Also, keep in mind that cell capacities contain a little chemical reserve potential as protection against reversal. So, even if you do dim it out almost completely, the low cell is likely to be recoverable.
Overcharging (and the heat generated) is the more likely problem you'll face. Although, with a smart charger, this possibility is almost eliminated.
Wilkey