You should use raster scaling if there are Copydot images contained in the electronic artwork. Because Copydot images are already screened, they are prone to artifacts when they are scaled. Normal (vector) scaling of Copydot will simply remove pixels as necessary, all in the same row. If these pixels are removed at the edges of the same rows of halftone dots, visible artifact lines will appear when viewed at normal viewing distance. Raster scaling randomizes the removal of pixels in both the X and the Y axes.
Left: scaled copydot shows noticeable lines where dots are flattened, due to pixel removal; right: raster scaling randomizes pixel removal, and prevents visible artifacts
Raster scaling is advantageous when used for Copydot, but has a side effect with lines and edges of geometric shapes. Because raster scaling randomizes pixel removal, it also has the effect of feathering the edges of lines and solids. These feathered pixels don't normally transfer to plate after the mask is exposed to UV light. However, some flexo printers find the feathering objectionable when viewing 1-bit TIFFs, and consider this a loss in quality.
Lines have been feathered with raster scaling