I've seen them deployed in flight on a 737 IIRC.
If you are asked to get a move on by ATC before starting a descent, or if you are given a late descent, why not use them?
Ideally probably would not want to as its effectively 'wasted' energy, but if it gets the desired effect for the situation, I don't think anyone has made a 'mistake'.
Disclaimer: I've never flown anything with spoilers on.