some you cut back to the ground in the spring some you cut back a bit in the spring some you don't touch at all
if it flowers early spring don't cut it. if it has dainty flowers and flowers in early summer then cut to ground in about feb or march. if it has big flowers and late eg now them trim it a bit in feb/march.
but they will come to no harm if you get it wrong. If you leave them then the ones that are best cut back will just get a bit leggy adn flower higher up next year. if you hack back and they don't need it hten they will either not flowr that year or flower later closer to the ground. if you can find the name of it that would help.
It has big flowers now, so will leave 'til Feb or March - thanks people! I've been chopping it down to the ground each autumn, oops! It always comes back ok but each year I wonder if I'm doing the right thing - and apparently I wasn't
whoops got in a muddle, if you want to prune yours you are quite right do it down to the ground, it's the earlier flowering ones that you cut back less far. But you don't HAVE to prune them at all. Pruning keeps them in the space you have for them, and means they flower further down and not just on the tips miles up in the sky.
Spring is better for pruning as when you cut them down they start producing new growth, and if they do that in the winter it may get a bit cold for them.