Scratches are caused by abrasive particles that are pushed against the paint. Here are some sample scenarios:
1. You wash your car and have a brand new (perfectly clean) car cover that has a soft surface. You should be fine the first night you use it. Even if you drag the car cover over the car, you will be fine because there's no dirt or rough surfaces involved. But wait until the next day...
2. You drive your car to work and back and leave it on the street all day. Now when you put the car cover on, if you drag it, you might rub the dirt that's sitting on the paint surface into the paint and cause small scratches. And now the car cover is a bit dirty. Over time, the car cover will pick up more and more grit from your car, so if you put it on a perfectly clean car and drag it or put any pressure on it, it will probably cause small scratches.
If you have a dark colored car, there doesn't seem to be a way to prevent minor scratches because it's impossible to use perfect technique every time you put the car cover on and take it off and you can't wash your car and car cover every day. So grit will build up on the car cover and car.
If you have a light colored car AND you use good technique AND you wash your car regularly AND you wash your car cover regularly AND you get your car detailed regularly AND you don't mind minor imperfections in your paint, you'll probably be fine.