It blooms on new growth. If you don't mind losing the blooms you have you can now. I hard prune mine in early Spring. The harder you prune the bigger your blooms will be. If I were you I would enjoy the blooms and hard prune it in late fall. I enjoy the dried blooms in winter thats why I wait till Spring to prune.

I wouldn't prune now nor later this season. Pruning DOES stimulate growth and late August is bit too late in the season to encourage that response. If you are worried about the weight, you can always deadhead or remove some of the flowers to lighten the stems - this is not the same as pruning. And if you prune back the canopy hard in late winter or early spring (the preferred pruning time), there is little concern the stems will be permanently bowed.

Thanks for the responses. I think I already knew this answer and I will follow your advice and just deadhead the flowers - although I do enjoy the winter interest of my dried hydrangeas - I don't want to weigh the branches down anymore. I will be less timid next spring. Thank you both!