There are a number of people who say that you can sow seeds at any time if you have the right environment. I'm not quite so sure, but that's probably because I don;t have "the right environment".
So please bear with me if you already know all of this, but my expereince over the last few years of growing maybe 50-100 species a year suggests to me that for my situation the best time to sow in February to March.
I have my propagator in the greenhouse, which itself is kept at about 6C minimum. The propagator is one I made myself, basically a box about 24x24x3 inches, half filled with sand, then a soil warming cable imbedded in this and finished with an inch or so of sand again. This is enclosed by a clear plastic lid, standing about 10" tall - its a commercial one, and I made the box to fit. The soil warming cable is wired via a rod thermostat imbedded in the sand, and the temperature is set to 23C.
I have 4 25 watt energy saving lights fixed above the plastic lid (simply to save having to sort out water resitsnat connections etc), and these are wired to a time switch on from 7am to 7pm.
Then I sow each species into 2" square pots, filled with sterilised compost, seeds just lie on the soil, and then well watered from the bottom, and put into clear plastic bags tied at the top. That way a humid environment is maintained pretty well.
I think that maybe my soil warming cable isn't powerful enough to always maintain the temperature if it gets really cold, and that the extra light levels from spring coming as well perhaps help.
I think that's why I find early spring best. I also have a decent enough time for growing them on and transplanting them to setle well for their first winter - which I don't treat as grown plants. I keep them warm, some water and try to grow on slowly until spring again when I then treat them as other young plants.
I hope this helps, its what I do and it does seem to work, but many others get good results by doing other things!