I haven't used them on watercolor pencils... I guess just 'good ole expermentation' would work...
Yes, on the emery board. I've used them and actually like them better. I find the cute little sandpaper blocks
bought from the art stores are okay but the papers tend to curl up at the far, unglued end.
Also very, very super-fine grit sandpaper from the hardware store works fine. You can buy those by the sheet very economically. Basically you can make your own sandpaper block by cutting a sheet down.