I have different grit preferences depending on which bodies I am sanding. For a small-particle porcelain, I can't go less than 150 grit without instigating blemishes in the final product. With the stoneware I use, I can go as coarse as 80 grit. Take note that sanding for high-fire applications isn't as effective, as high-fire clay bodies will tend to flux and resultantly change their surface due to the shrinkage. I do agree with other posts here that there is a health hazard, however, in order to meet the standards that I hold for certain lines of work, I find sanding to be integral to creating flawless surfaces. Also, I am generally sanding surfaces that will tend to remain bare at the end of the firing, so there's no relying on glazes or otherwise to average out the defects. I tend to do a post-fire sanding for this type of work in addition to the bisque sanding.