The best flavor in mint is right before it flowers, but the leaves are still useable, and the flowers are too! They taste just like the leaves, but a little milder. Bees and all manner of other pollinators love them, and they're a good way to draw them in. You may want to clip the flowers if you don't want it scattering seed, though, mint seeds can be very unpredictable, usually undesirable, (particularly if yours is a hybrid to begin with), and rather invasive.

Thanks!