I'm located in a SW Chicago burb and put my 1 - 2 week old seedlings
outside in a coldframe consisting of 4 mil plastic, PVC tubing and dyed
water bottles (to control temp flux at night). In years past, the
little guys have survived lows into the mid 20's, but I do toss a
blanket over the cold frame at night to help out. Haven't lost any
to frost doing it this way, but do have indoor ones just in case. So
far they have been outside two weeks are are small, but stocky.
I have an uncle in the Kalamazoo (MI) area that plants on Good Friday,
and yep he has lost tomato plants over the years. He has saved some
though by putting a sprinkler on at night and letting an ice shield
form over the tomato plants. I have no idea how that would save
them, but it does LOL.
Long story short, lows in the 40's is fine. Lows in the 30's, yah
better protect them - even a 5 gallon bucket may do the trick