What's the best way to cut back my crocosmia montbretia?

I wait until the early spring and then clear away the dead leaves. The new shoots are just showing themselves usually. I split the clumps at this stage, too, if necessary.

Yes, I do the same. Crocosmia grows from a corm so, like bulbs, following flowering it needs the leaves to build up the food store ready for the next year's growth ..... if you cut them down now you will not have such a good display next year.
During the winter I am sure the dead leaves act like a protective blanket over the corms.
If the clumps are out-growing their space or the previous year's display was disappointing due to over-crowding they can be dug up and split in the Spring at the same time as clearing the dead leaves.