I personally wait until it starts to leaf out and then cut back only the part that didn't make it through the winter.

I cut all mine down in mid-March to about 6". Last year I didn't cut down one of them til end of May and it was way behind in growing new stems. Mine die back to the ground each year so that's why I prune them so early.

tess

I wait until they show new growth. Two which I have located on the south side of the house die back to the ground every winter, but I've discovered that the southwest one in the backyard does grow back from the stems/branches/trunk. It is getting rather big and I'm going to have to do something about it - not sure what... They are all the same - lilac with orange center.