Should I prune or deadhead the seed heads on my Clematis?
Whether to deadhead or not is a personal decision. Many find the seed heads to be very attractive and leave them on until they mature and drop in the fall. Keep in mind that producing both flowers and seeds requires resources and can drain the energy of the plant. Well-established and well-fed plants produce both of these with ease. However, it is probably best to deadhead young vines to help them get established faster.
With some of the more vigorous varieties such as Terniflora (AKA Sweet Autumn Clematis), montanas, or most species, deadheading is not required. If you deadhead the early-blooming, large-flowered Group Two cultivars you will usually get a sparse second blooming and more growth. If you intend to grow new cultivars or species from seed, let your best performing vines hold their seed heads until they have matured.
It is up to you and what your aim/goal is.