The co2 shouldn't change the temp of your grow room. And like tplat said, plants need extra co2 to handle high temps. Thats the main reason why outdoor plants handle the heat so much better than indoor plants. It's all that fresh air.
interesting question. but really it depends on the method that you create your co2. a burner system will increase the heat in the grow space. If you have a tank on a timer that is adding CO2 from a tank that is under pressure it will cool the room because when you take pressurized gas out of a pressure filled environment it gets cold, that is why if you dump a CO2 tank it gets frosty. If you are using some sort of more natural way of generating CO2 like a organism creating it (yeast,etc...) then heat again. Now the good news aside from the burner method the heat/cold that is produced is so unnoticeable that it will not raise the overall temperature in the grow. we are talking %tages of a degree.
Thanks! I got a tank fyi... also I got a lil teeny tent (like 2 x 4) figured id do co2 cause I had a homebrew tank laying around...