I know this is an old post, but this information might be useful to others thinking about this.
Car covers.........love or hate, they are like marmite!
Yes they can cause scratches even the best ones, I have used one on my own car, to which I'm not overly happy about but it was better than leaving it for a few months cover in bird poo.
Honestly if you're looking for protection these are better options:
Car port.....expensive option and not always possible.
Rent a garage.....again not always an option.
Prevention and protection = get it covered with a ceramic coating by a professional and keep to a fairly regular maintenance plan of washing. If you cant stretch to a ceramic coating, a good application of a quality sealant and top it up with a regular waxing.
Just my view, car covers are fine(ish) for one offs but they will/can still cause some paint damage that will require correcting.