Greg,
Most stalks will have some seed. The heuffs and things that are totally or predominantly tectorum, marmoreum, montanum or arachnoideum will have LOTS of seed. The hybrids that are part arachnoideum X a non-cobweb will have much less seed in general, sometimes none at all. My 'Greenwich Time' which is a montanum x calcareum cross has almost no seed. Hybrids from tetraploid strains X diploid ones tend to be nearly sterile, such as 'Greenwich Time'.
Hope that helps.
There are a few others like 'Aymon Correvon' (alias 'Purdy's 90-1' and 'Engle's 13-2') that I have never obtained seedlings but I do get things that look like viable seeds. I'm trying again this year with 'Aymon Correvon' as I really like that combination of grey and velvet and would like to work with that combo in my seedlings too. Just think of something like 'Polly bishop' in velvet.
Kevin
