In my experience for potted plants, Fall is the best as long as they aren't forced into total dormancy because of too cold soil temperature. I've pretty much done this at all times of the year and I get my best root growth by doing it in the Fall. By the time Spring rolls around and the soil temperature starts warming up I have a whole pot full of new roots and the growth that occurred during the winter takes off like a rocket.
It seems the plant spends the Winter growing roots rather than leaves. Plants I've cut back like this in the Spring do not seem as nearly robust by Summertime as Fall plants, maybe because the root system isn't extensive enough to transport a lot of nutrients to new growth.
