I've got a pearl white Z as well and I cover mine with a generic car cover I bought from PepBoys for $50. I don't remember the brand, but it was one of two they had in stock.
My Z stays outside in my driveway every day under two big trees that the birds LOVE to sit on. The cover has taken somewhat of a beating between the bird $hit, sap and pollen, but in the 2 months I've had the car, the paint has not been affected whatsoever. Granted, I wax the car monthly and wash the crap off the cover weekly (i.e., every time I wash the car itself).
And like Alexus said, it's impossible to keep the car perfectly clean anyway...unless of course you put your car in a vacuum-sealed box and leave it there forever.
