I never cut back my Hydrangeas unless I want the lovely colour faded flower heads to bring in doors for decoration, (hang upside down in cool airy room / garage and when dry, spray gold, silver, red etc for Christmas dec's.
Early spring soon as you note the tiny new side buds are showing, that's when I begin to cut my plants back, always make the cut on a sloping slant away from the bud , cut just above a bud and make it an outward facing bud, this way your new growth will be open (aim for like a tea cup shape) as this allows plenty air to get into the shrub and sunshine / light too.
Hope this makes sense to you and it helps, Roses and most other shrubs benefit from this type of pruning IF, they in fact require cutting back, Rhododendrons, Camelia's and this type of shrubs don't require any pruning unless to remove dead wood or cut down to size.
Hope this helps you out and good luck. WeeNel.
Do you know what type of hydrangea you have? Quite a few of them bloom on old wood and if you prune them now you will be cutting off next year's flowers. If yours is one of these, then you also can't prune it back in early spring since that will cut off the buds as well, you need to wait until after it blooms next year if you want to cut it back. Only pruning you can safely do this time of year is cutting off faded flowers as WeeNel mentions.
There are some types of hydrangea that bloom on new wood and could be pruned now, but unless it's gotten too big for its space and needs to be trimmed there's no need to cut it back before winter. If you're in doubt and don't know what kind you have then I would definitely leave it alone--none of them need to be trimmed back this time of year and many of them it can prevent next year's blooming so better safe than sorry.