I use a cover on my Boxster and 911. There are a lot of options (materials, thickness, reflectivity, water proof, etc) so you'll need to decide what exactly you want the cover to do first and then select the type of cover to meet those needs. Also, you tend to get what you pay for in a cover so expect to pay a fairly decent amount of money for a good cover.
For example, I have the factory cover for the Boxster (which is nicely lined with a felt like material but it isn't really weatherproof) whereas my 911 cover is a shiny reflective material with no lining but is generally waterproof.
Neither of these cars are concours condition so even if they were to suffer from a little bit of clear coat scratching it definitely won't keep me up at night.