I don't think it's a good idea to give your dog marshmallows. Yes, there's nothing toxic in marshmallows for dogs but as
About Doggies states sugary treats aren't really good for dogs. If one would constantly give his/her dog marshmallows or other things high in sugar content, then it's very likely to lead to a number of health problems (dental cavities, obesity).
Remember the times dogs lived to be 20+ years old? That was before we started to feed them commercial dog food and treats and snack food made for humans.