What CMYK values should I use for the brightest highlights in an image?

I usually set the whitest part of the image to C5 M4 Y4 K0, but this is making some product shots of glassware (ie wine glasses, glass trophies) look grey.

Does anyone use a different CMYK mix for the highlights? I don't want there to be no colour in the white area as it will look blown out.