Ukraine isn't about to waste missiles that it would likely no longer get as a result by using it on soft targets of zero military value. Now if Russia happens to be using towns as a shield for their own ballistic and MLR systems then it becomes questionable, but justifiable if not attacking risks Ukraine's defence.No Russia will start long range missile attacks to goad Ukraine into using their newly purchased own long range missles onto Russian towns for the propaganda.
I'd imagine that Ukraine would prefer plausible deniability through more covert/surgical options instead of blanketing an area with rockets.