Should I prune off the seed heads before the seed gets a chance to form? And when can I take cuttings so I can propagate?
Pruning and propagating clematis...
The seed heads are part of a Clematis' charm. If you grow them from seed, they will not be identical to the mother plant. You will get a variation of the parent. It also takes several seasons of freezing & thawing to crack open the seed so that it will germinate. Once it does germinate, it will take several years before the plant is mature enough and the root system is developed enough to bloom. That is why Clematis costs more $$ than other vines.
Yes, you can propagate them from cuttings. The best time for propagation is in the Spring. You can still propagate them now, but you will have to keep them moist.
Here is a link that will give you more information of propagation.
http://www.briansgarden.com/questions/020320r.html
It's b/c of those reasons I'm not interested in the seeds. Will cutting them off make more flowers like dead heading a rose would?
You will probably get a 2nd smaller flush of blooms if you deadhead & also fertilize them again. Stop fertilizing at the end of Summer.
Thanks!