Riser blocks would work but you would need to align the axes each time to make them perpendicular. The other common approach is to swap the Z rails and carriages over so the carriages are fixed to the Y axis plate (at the bottom) and the rails on the Z axis plate slide up and down. That keeps the minimum offset for all Z positions. Optionally you can also then have a riser block on the bed to bring the work up to the spindle as required. Depends how big the work piece is and if you would want to skim the bed riser before use to ensure it is flat relative to the spindle.