Not really (they can also come off purlins), just needed to get my head around it, not helped by your reference to a "valley truss", by which I assume you mean the valley rafter?
Difficult to describe it all in words, so perhaps it might help if you take a look at this...
http://www.familyhandyman.com/DIY-Projects/Indoor-Projects/Attics/Attic-Remodeli...It's a US site, but all the references are there and it gives an excellent step-by-step view of the different stages, including the laying out of the valley rafters.