Yes, the magic eraser is pretty cool - but it often cuts away unwanted colors (especially if you're using white as a backdrop). That's why blue or green screens tend to make life easier - less likelihood of finding those bright colors inside your object.
I also mess with manual levels (Shadow/Highlight under Adjustments is good too). However, I'm not sure what you're calling manual white balance? Are you talking about curves?
Condottiero - Freelance Multimedia Design