I dont think it really matters when you winter-sow tomatoes. They don't sprout before the weather warms up, and they don't need a freeze in order to germinate, so you can really do it pretty much any time.
My spring probably comes a lot later than yours (zone 4 WI), but my experience with winter sowing tomatoes is they come up much later and are much smaller than I want them to be in May. My tomatoes started indoors under lights are sturdy 12-18" tall plants by mid-late May. The one time I winter sowed tomatoes they were still 2" sprouts in Mid-May. My season is to short to begin that late.
