We have a ranch house with painted cedar shake siding. As we prep to repaint, I am wondering whether we need to caulk the places where the siding has cracked vertically, before we paint.

Is the siding really split shakes or is it sawn shingles? Pics would help.

If the crack does not line up near a seam of the shake behind it you do not need to caulk. If it is even close to the seam behind it you must use a paintable caulk.