That's up to you. If you do the treatment now, then you'll need to wait longer for it to work. The downside is that the pool will be at low pH for a longer period of time. Though normally not good, the same effect on reactions that are slow for ascorbic acid removing stains also happens for reactions of corrosion.

If for nothing else than the peace-of-mind of having the process go faster and where you can monitor it more closely, I'd personally wait until the water warmed up a bit, but that's just me.