I planted these so I could use the blooms for a wreath, but I haven't been able to use one flower as of yet. Hydrangea macrophylla 'Nikko Blue' is a mophead type that is extremely popular due to its vigorous nature and large flowers. Indeed, the blooms from 'Nikko Blue' are well suited to drying for a wreath. Your intuition about timing serves you well, although it's only half the answer. The other half is how much you cut.
Hydrangeas are shrubs that respond well to selective pruning or no pruning at all.