A cover inside is not a bad thing, as long as the car is properly cleaned before putting the cover on. Using a cover outside or putting a cover on your car after driving it without washing it will do more harm than good!
2011 E90 M3 ZCP - Individual Moonstone/Individual Amarone Extended/Individual Piano Black With Inlay:LINK!!!
1994 Euro E36 M3 Sedan - Daytona Violet/Mulberry:LINK!!!