I would say to go ahead and sow the seeds now. The more time the have to make their way into the soil, the better. They likely won't germinate until next Spring, however. I wouldn't stratify them and them, and now sow, as it seems too late in the season for this.
I order my native seed from Prairie Moon Nursery in Minnesota. Their website has germination codes next to type of plant. For Asclepias, they note that 30 days moist, cold stratification is necessary, fyi.