Only when it is dormant. If it's in active growth, the cuts will bleed badly. I suggest it's done immediately after the leaves have fallen

I would say wait till February. The sap will be low and any dead branches/twigs will be whiter looking, so you can clearly see where to prune back. It will also be going into leaf within 2 months of Feb, so it will bounce back and the coldest winter weather is behind.

The general advice is not to prune Acers unless you really need to.