Perennials and many annuals should be fine now or even a month or so ago. Things like zinnias I'd wait till a couple weeks before LFD -- they sprout relatively fast and can't take the cold. Many direct sow zinnias. I just do them in containers so I know what they are and can plant them where I want them.

I live in Ottawa and I start certain seeds (very hard ones like Cannas) in January. I consider anytime after we have had our first snow to be fine for winter sowing. You can expect your first sprouts sometime in April/May depending on where you place your containers. I have mine in shade to prevent drying out or 'frying' if we should have a temperature spike. I start my tender annuals once I see sprouts. Of course you can winter sow hardy annuals like petunias, daturas, and veggies like lettuce, chard, etc. in the regular winter months.

Thanks, Northerner! Thanks for your sharing. It clears my worries.

I like winter sowing but don't know anyone doing winter sowing in Toronto/Canada.
My first sprouts was May last year. I kept thinking why my sprouts was so tiny in May. Would they miss the blooming time? Well they did bloom, Zinnia bloom in July, Snapdragon in July/August, Petunia in July also.