Pondy, I too sow seeds this time of year. Sometimes they germinate too early, but more often, not. Amaranthus and cleome are two annuals that have done well for me. My black eyed susans and cone flowers often self-seed. I just help them along, or move them to another area by fall seeding.

I usually sow in extremely early spring for the perennials. Right after the beds can be cleared down to the bare dirt. The cold stratification of the wet and very cold early spring really can do the trick. This works well for me with columbine, poppies (both annual and oriental), dianthus, nasturtium, echinacea and I'm sure others.

Fall would probably work as well as long as you're not going to rake out the beds, moving the seeds!

Many perenial assess require stratification so fall or winter sowing is best. Ideally you would have prepared the bed and removed any perennial weeds last fall, but I have also broadcast natives into the meadow with success without any type of preparation ahead of time. I think the best time to actually do the broadcasting of the seeds is during a snow storm, because then the birds don't eat them.
Catrina