For germination, the seed likes some bottom warmth, like a heat mat.

Once emerged, first set of true leaf or earlier, off the heat.

No moving around in and out. Continue your grow in a cool room or garage 55-60º or what
similar temps you have that remains steady.
(my grow room is 60-62º)

You could try an un-heated room or your garage not near the door.

You should read through these pages as many are giving their various experiences.

you may be thinking of 'hardening off' ?...even that process should be less high-low
in-and-out temps. Though sometimes unavoidable with unpredictable weather.

Thanks oakley, that clarifies things. I guess I assumed it was like hardening off. I jave read through all of the posts and think I am on target now. I don't have a likely spot that stays around the 50* range so would putting the plants in a shaded North facing window with it cracked a bit work for the cold treatment for a month or so?

I don't move mine in or out. After they germinate in the 68F main level of my house, I just keep them under lights in the basement where it stays in the mid to low 60's. This creates very slow growth and nice, thick stemmed seedlings by the time they are ready to transition outside in mid-to-late May. Lights kept down low over the tops of the seedling is also very important.

I started seeds 4th of july last year so it was already hot out.

I am so glad my Tomato Plants are doing good by "Mother Nature Cold Treatment" as our snow melts, Amen!!

They really look good.