Here is the thing. If you sow plants that are not frost tolerant too early, and they germinate, they could be killed off by a hard freeze later on. The containers will protect them from a light frost, since they are like little mini greenhouses, but not from temps in the teens or single digits.

Poppies are pretty cold tolerant for an annual, so those would be okay, but in zone 6, I would wait until the beginning of March to sow the African daisies and sunflowers. In my zone, annuals will be sown in late March/early April.

If there are any other specific flowers you are wondering about, just post the list. There will be somebody here that has WS it, and can speak from experience.

Bonnie

I agree with Bonnie but I wanted to add a note about Poppies. Last year I WS'd poppies and direct sowed also. As much as I love winter sowing, I have to admit the direct sown poppies grew and bloomed much better than the transplanted ones. That doesn't mean I would never try it again. In fact, I just sowed a jug of red corn poppies for my mother a few days ago. As for hardiness, mine from last year reseeded like crazy and have been completely unfazed by cold weather. Our coldest morning so far this winter has been 9 degrees.

Mechele

Wubba
I have always been concerned that starting annuals early will only lead seeds to increased rotting. I have always had best results starting annuals or perennials that do not actually need cold period to germinate later on in the season, like March- April for my zone. Then they will germinate in a matter of couple of days.