The timing of pruning with Penstemon is crucial to their success. They should be pruned in Spring when the new shoots are 3-4" long. This could be in March, April or May dependent on where you live and the Spring weather. Here in the midlands we generally do ours around Malvern Spring Gardening Show time, i.e. the first week of May.
All of the summer growth should be left on the top of the plant in winter as this protects the crown, any new emerging shoots in Spring and give the roots something to do in the wet winter. You can take the top of the flower spike and seed heads off to tidy up the plants and stop them rocking so much in the winter winds.
Once the new shoots are 3-4” long, it is safe to cut back all the old stems to a new shoot. This can either be to ground level if the new shoots are form there or down the stem to good new growth.