Increasing the flatness value improves the chance that your clipping path will print properly. A flatness value of 8 will not generally be noticeable when printed on an imagesetter. You may notice it on laser-printed output, however. A clipping path over a vector-based blend results in a very complex PostScript routine. Blend simple unstroked shapes and avoid excessive steps in the blend.
Large masks are more difficult to print than smaller masks.
Consider rasterizing or converting the graphic to a program like Adobe Photoshop and then cutting the mask before placing your graphic in the page. Never scale a clipped graphic after you have placed it in your page-layout program. This may cause unexpected output.