@Becca1264 A true service animal is not a pet. They are trained to perform certain tasks for their owners, therefore are trained not get onto couches, beds etc--unless the owner requires the animal's help while in bed or on the couch. A friend of mine is a quadriplegic and her service dog will retrieve items for her and bring them to her while she is in bed--usually, Sunny(her dog) will place her front paws on the bed so that my friend can take the item from her. I have never seen Sunny lounging either on the bed or couch; she is trained to always be by my friend's side.
That being said, I don't think asking to cover couches, etc, is in violation of the ADA laws--at least I can't see anything that would suggest that. I feel that it could be perceived as a little offensive though-especially when you are dealing with true service animal.
We also have a strict, "no-pet policy", including service animals because of our disabled grand daughter' severe dog allergies, and I can definitely see both sides of the coin.
You can, however, ask potential guests with a service animal what tasks the animal is trained to perform if you are concerned whether you re dealing with a "real" service animal. Unfortunately, there are a lot of people out there who falsely claim that their ill-trained dog is an assistance animal, making it all the harder for those who are legitimate.