Pruning recommendations for hydrangeas differ as to the type of hydrangea. The excellent hydrangea website, hydrangeashydrangeas (http://www.hydrangeashydrangeas.com/index.html) has what I feel are the best, simpliest and clearest instructions for pruning any type of hydrangea. I particularly like the part where it is suggested that macrophyllas (most mopheads and lacecaps) and oakleafs do not require pruning to thrive and bloom and are best selected and located where they may be allowed to grow unimpeded. Too often, gardeners feel the need to control their plants and think that pruning should be an annual (or more frequent) routine to keep plants in check and to a size the gardener feels appropriate.

Here is a link that might be useful: my favorite hydrangea pruning guidelines