We are in zone 3. We usually trim after first frost. How we trim depends a bit on the plant. Eg: roses get cut back to a ball about a foot in diameter (and wrapped – some go in the garage if in pots); while blackberries get cut down to the ground if they produced this year – or they get wrapped as they stand are if they didn’t (they have a two year cycle, but rabbits fuck with them in winter); we have a decorative tree called a “smoke tree” which we trim back every year so it doesn’t grow too large.