I think it's around 40 degrees, if they are properly hardened off first. The problem with tomatoes at a low temperature is that their growth will be stunted. They may not die, but they won't grow much either. I read somewhere that they never seem to recover from it either. If it were me, I'd leave them inside until May, then I would only put them out on warm days and then plant around Memorial Day, if the weather looks good for that.