I generally wait until the leaf buds start expanding before pruning out dead wood. The few that I have that bloom on new wood (excluding 'Endless Bummer') - I will shape them a bit if the stems get really long because I know they'll just bow over when they bloom. I've always heard to just leave hydrangeas alone except for the dead wood thing. I did hear a podcast mentioning that once the shrub is in bloom that the non-flowering branches can be cut back so there's a more dramatic effect of more blooms/less foliage. Have never tried that one since I wondered if the reduced biomass would be an issue for the plant. I've never documented the before and after but then I'm not much of a picture-taker.
