Hydrangeas

i dont do mine until late spring and then only deadhead or down to the first beds after the spent flower. I reduce a few down to the lowest bud but those dont flower.

Hydrangea macrophylla flowers next year on wood produced this year. So, if you cut it off, no flowers. Normally the advice is to leave the dead flowers on over winter as frost protection and to remove them in Spring. At the same time you would remove any dead shoots. If the bush is overgrown and the centre congested, then you can remove those branches growing up through the middle.
Hydrangea paniculata and arborescens flower on new wood so they may be pruned as hard as you choose, again in February.

I just leave them alone-- dead heads especially as they are supposed to protect the new shoots from frost, if you cut them back now they probably won't flower next year, most books say if you need to prune for size or shape etc.just up to a third per year

Basically, don't touch them until new growth starts, probably March/April depending on how warm it is. Then take off the dead flowerheads, nip off any dead tops not showing growth, take out any dead stems.