I'd do it early spring if you need to when you can see if any branches haven't survived the winter, but its not necessary unless you want to tidy them up or reduce the size. If they need shaping just take out crossing or weak branches and any that are growing along the ground where you don't want them.

Once in a while, you can be brutal and cut them down to a stump if you want to - they grow back well. I did this to an ancient inherited one, and by the following year, you would'nt have known it had been done