What sort of plants are you interested in starting? Annuals or perennials? Flowers or veggies? The one time I tried starting seeds this early, it turned out to be a huge mistake! But I grow mostly annuals. Now slow growing perennials might be worth considering, but so much depends on the type of plant. If you have a shiny new greenhouse, I do know that it's hard to wait to start something!
And I'm sure someone else will chime in shortly with other thoughts.
