your mileage don't really matter to much as your can can get covered in contaminates whilst parked up if not in a garage or sheltered,

have a quick feel of your paint and see if you think it needs it, best way to do this is get the plastic wrapper off a pack of fags and put that on your fingers as this will heighten your senses in your finger tips so you can feel the contaminates even more,

the HD paint cleaner is a great product though, i used it for the first time the other day and was very impressed,