Yes, ideally the soil in a cold frame will be warm enough to keep the seedlings cozy if the temp should drop to freezing. Some people have additional elaborate insulation below the soil line to further concentrate the heat.

But if your cold frame is on your deck, it should be easy enough to throw some blankets or quilts over it if the temperature is marginal, or even move the seedlings back inside for the night if you're afraid of a hard freeze. You could also put in some plastic jugs of really hot water to bring up the heat before you cover up.

But don't forget to open the top and provide ventilation for sunny days, the temps can rise quickly in an enclosed area. You might want to put a thermometer in and check it a few times a day to see what temperature extremes you'll need to regulate.

I have used what Mel (from 'Square Foot Gardening' fame calls the 'sun box'). Unlike a traditional cold frame you build wood panels in a box shape and these pieces can be stacked with as many other pieces for the height you need. They are easier to stash away when you don't need them after your area warms. I put mine underneath my patio deck. You can see them being used here in my photo:

My tomato plants grew fast and hard under my light setup and outgrew the sun box because of the sheer number I had going, so I used my clear-glass patio table and a thick plastic bag that came off my sofa as a makeshift greenhouse. It worked fabulous too!
{{gwi:294515}}From 032409

That's pretty slick bencjedi.

The ave temps these days are 61-65F in the day and 45-48F at night. I'm wondering if I really need a cold frame, especially if I would be lifting the lid during the day so it doesn't get too hot. Could I just place the tomatoes on the deck and cover them at night? Secondly, would there be an issue with this if it rains?

thanks