I always scan at 100%, and adjust other parameters (ppi, etc.) to fine tune the actual size of the image. If I ever needed it larger, I would upsample in a dedicated program like Genuine Fractals, not when I scan. However, I have never needed to scan at more than 100% at 4,000 ppi for a desired print.
As a professional graphic designer, I have to say this isn't good practice. No application can create detail in an image that is not there. Scanning a MF neg, for example, at low resolution and then blowing it up is something like taking a picture of a MF photo in 35mm and blowing it up. You should scan at the highest resolution practicable for the application you intend. If you need a higher resolution later, it's far better to rescan it at a higher resolution.