They're a winter-dormant genus, but I am not sure when the best time is to propagate them. Someone else will have to help with that.

Here is a handy list of many dormancy periods:

Here is a link that might be useful: www.succulents.us/dormancy.html

Later spring is the best time - that way they have most of the year to settle in.

Semps seem to grow in all but the hottest weather, whereas cold-hardy Sedums seem to like that the most.