Can I prune loropetalums and if so, when?
The growth rate of loropetalums (a member of the Witch Hazel family) varies considerably by variety. Some stay low, and more horizontal in shape, while others are taller and more open. Loropetalum, with variety names like Plum Delight and Razzle Dazzle, have become popular landscape plants.

How do I prune my purple fountain grass?

If you prune it, you will be cutting the plumes off and that is the pretty part of the plant.

Be very careful when pruning Weeping Yaupon Holly Ilex vomitoria 'Pendula' or you will loose the prized look of the plant.

You may not want to bother removing the flower-bearing stems on many types of salvia that are grown for hummingbirds.

Fall-blooming salvias such as leucantha should be pruned during the summer to keep them compact, reducing the need for staking. To make the job easier, use hedging shears, and remove only the spent flowers and a few inches of stem below. Any of this type of pruning should be completed prior to September 1, since flower buds begin forming about that time.
Note from Parker County Master Gardener: As for the leucantha, it has to be cut back completely to ground as does Victoria blue salvia because it dies to ground.

Nemerosa blue salvia 'May Night' meadow sage and they need deadheading/pruning.