To cut down or not to cut down..?

OK, here I have a question.  I am usually a pretty solid green thumb, however, I am constantly perplexed by this simple question.  Hydrangeas, the beautiful blue (or pink, depending on the acidity of your soil).  When the season has ended, cut them back completely, partially or just leave them alone?  I have to admit, I’ve tried all the above and have either caused them not to bloom, ended up with a lot of stalky wood or killed them completely.  I live in the northeast and have some severe winters, nonetheless they do thrive in our zone.  I have glorious white hydrangea bushes, lace-cap hydrangeas (that do very well – except when my little deer families are having them for breakfast) and a very showy tree.  Any comments to share?