should i cut back my 5 year old penstemons

Never cut back Penstemons in the autumn - they can succumb to frost. If I were you, I'd take cuttings now, keep dead-heading until the autumn and then leave the last few flowers as frost protection. In the spring, look for new shoots at the base of your plants and if there is some, cut back to it. If you think they are past it when you look, then you will have your new plants from the cuttings to replace the old ones!

Spritz is quite right. And cuttings root really easily from Penstemons. If you're a novice, just put some stems in a jar of water - they should root like that and then you can just pot them up.

I do that, Sid - I get 100% success with cuttings in water, but not in compost, for some reason!