I press so that there's the least bulk. I use a lot of white and there has only been one occasion where my willy nilly pressing technique allowed the darker fabric to show thru. I think it was because the white fabric was thinner than I normally use. Except for this one time, I personally haven't noticed any show thru with lighter fabrics. Remember when you press to the light, there are 2 layers of the light fabric (top + seam allowance) that will shield the darker fabric.