Surely it depends what you're scanning in. If it's pencil or line work, scan in as greyscale. If it's a painting or photo, then scan it in colour mode.
I usually scan in at 600dpi and then reduce to 300. You will be guided by your scanner's software. Your 'format' will then be decided by what
you're going to do with it afterwards and what you save it as, etc. If it's staying as a sketch I stick with jpeg, if it's going to be turned into something else
then it might be a Tiff etc.