It really depends on what you intend to use the image for. For web display you need not use maximum resolution. To print really large you will. My workflow is to use Vuescan's Edit size at 300 dpi which I have found to be a good enough compromise for both web use and for the occasional print up to A4 size. Don't forget that image quality depends on many other factors, such as having a properly exposed negative, post-processing (and your pp skills) etc.