scientifically I have no idea if it would help.
however I dont see where it would hurt either.

I will say yes, because it will prevent that the ground gets frozen inside the greenhouse. My experience is not with a greenhouse, but with my little 4x7ft pond. I cover that pond with plywood and old shower curtains and prevent it from freezing. Actually I can let gold fish survive in there in 18 in of water. Adding several inches of mulch should be better.

Berndnyz, Thanks for the info. I have many ponds and will be trying your idea. Mostly I just leave the pumps running all year long and this keeps them from freezing but your way sounds like it will keep my fish warmer. I will update you on the experiment with the mulch around the ghouse. This is my second year of greenhousing and I'm already building one twice as big. Tnere is nothing like this type of gardening. The yields are fantastic, and I still have very large tomatoes that haven't even ripened yet. Believe it or not they are still growing. I had ripe tomatoes before anyone else and the plants just keep growing and growing. I will have ripe tomatoes for thanksgiving now that's incredible. No more deer, no more rabbits, and no more turtles.

If you have air infiltration around the base of your greenhouse, then piling dirt or mulch around it will probably help. I have a 20 x 24 hoophouse and this is what I'm trying to do to reduce air infiltration.

A preying mantis took up residence in my hoophouse--she's still alive. Every few days a handpick a grasshopper or a cabbage worm and she has a feast!

Three enormous tomato plants are still growing, as are several cucumber plants--the third set that I started. I doubt, but have hopes that one or two of those little cucumber fruits will grow up!

How big are your ponds? Should I consider adding a little pond?

It will keep the edges from freezing for sure. I mulched out to four feet from my hoophouse to a depth of one foot this year!

I use leaves to mulch my carrot crop every year and it works awesome. Lets see, Last Jan it was zero out with a foot of snow and I was harvesting my carrots as usual! The power of mother natures insulation!

The pond in the greenhouse is 3 ft by 10 ft and 3 ft deep in ground. A pond is completely up to you but I will tell you that mine serves many purposes. It stores heat, fish and no so hardy water plants. And also when I clean it out I use the fish waste water for fertilizer. So for me all my ponds are serving many purposes. However it is alot of work to keep 9 ponds up and running, but the fish breed with no intervention from me and the waterlilies grow like crazy. This all makes me money. Mostly I do it for hobby but I like to teach people and get them started doing the same things. It's amazing what God will provide you with if you just put forth a little effort. Greenhouse gardening is awesome. I love hearing the experiences of others. Thanks to all.

Boomantoo,
It depends upon the size of your greenhouse.
In university studies, about 2-3% of heat is lost through the perimeter. Here is a link: http://forums2.gardenweb.com/forums/load/strucs/msg0112120225762.html However, I believe that these studies are done on larger greenhouses or greenhouses in warmer climates or both.

I did some calculations using a smaller greenhouse in a colder climate here: http://forums2.gardenweb.com/forums/load/strucs/msg0312555932001.html I came to the conclusion that a 6x8 GH could loose up to 13% of its heat through the perimeter. That is significant and it would make sense to insulate the perimeter in some way.

You must distinguish perimeter loss from loss through the ground. Losses through the ground are essentially zero, which may not be intuitive, but is discussed in detail in one or more of the three links I've included.

Here is a link that might be useful: Another discussion