Any pruning of shrubs in the fall should be done after the plant is dormant in the winter. You don't want to encourage any new growth before winter. Pruning will encourage the shrub to put out new growth. However, if you prune shrubs the bloom on the previous season's wood in spring, like forsythia, you could be eliminating branches that might flower. The best rule of thumb for pruning flowering shrubs is to prune right after they're done blooming. Dead branches can be removed at any time.
