It won't harm it to prune now Ruby, but you will get less flowers. If you cut the lead buds off with the ends of the branches, the ones lower will take over and you may well get a second flowering later in the year to make up for it.
The general rule of thumb for anything flowering before the summer solstice is that they flower on old wood, so you prune once they have flowered. This gives the shrub time to make new wood that will flower the following year.
With anything flowering after that, they usually flower on new wood. You prune over winter or early spring, so you get a spurt of new woody growth for flowers.
With most shrubs, if you prune at the 'wrong' time of year the plant survives just fine. You just may not see many flowers. If you have an overgrown triffid that is taking over, that might not matter so much to you anyway
