Snowball (Viburnum opulus Roseum) needs very little pruning. You can thin out some older stems every couple years; remove older, bigger stems at ground level with a lopper or pruning saw. The best time for that is early spring. If what you want to accomplish is a shorter plant, wait until bloom is complete, then prune stems back to just above a side branch or set of leaves, removing no more than half the stem length.