The best time to root prune is when the plant is in dormancy after all foliage has dropped. Pruning in spring after the buds have started opening can delay the plant progress.
I am not expert on it, but I think that for areas with moderate winter temperatures pruning in fall may help develop some roots during dormancy that can help in the spring (comments welcome on this). I am in Zone 5a and prefer to root prune in spring before bud break.