I do it all the time, only way to protect on big downside

In general, if stocks moves closer to or pass your short side of the spread, it is time to consider roll it down. But I also use elliot wave to see if the current wave down is close to completion, in which case, I sometimes would take a chance and not roll it. 

But in a clear downtrend (ie, still in the middle of wave 3 down and not even seeing wave 4 and 5 yet), it is a good idea to roll it down