Crumble

Hey guys, i just recently bought a car cover from www.carscover.com for my IS and was wondering when is the right time to use it? I was planning on just throwing it on the car whenever...but my friends are telling me that i should have the car washed or detailed before i use it. I do have an aftermarket paint job so im trying to protect the paint but keep the car clean as well. The cover is great and i really want to use it...but i dont want to have my paint scratched or such...