I save the keywords, description, and copyright information in the image file while in PS. iStockphoto extracts the information from the file when I upload it.

Personally, I ignore the ratings completely. I don't think that they matter in a search unless the person doing the searching explicitly chooses to sort by rating.

I definitely would NOT delete low rated images just to improve your average rating. All that would accomplish is to reduce the number of your images available for purchase.