Generic advice is not going to work for you :-) First, there is no generic pruning advice applicable to hydrangeas - you need to know what type you have in order to know when, or even if, you should prune. Second, 'snowball bush' is a common name that is applied to other types of shrubs as well as some hydrangeas, primarily viburnums. It is really necessary for you to ID the plant in question before undertaking a lot of likely unnecessary pruning.

Pruning incorrectly (for the shrub) and at unadvised times is probably not going to seriously damage the plant, however it can impact its flowering ability and can alter habit and appearance unnecessarily. Get your plant ID'd first either by posting photos here or taking a portion to a good local garden center, then we can proceed with any pruning recommendations, if applicable.