In all visual styles lines crossing from corner to corner normally display on decals (but they don't show up when you render).
This can be a problem if you want to use Realistic Visual style for output.
You can turn the lines of by:
selecting each decal,
right click and select Override Graphics in View
then By Element...,
tick Transparent.
You will need to do this in each view the decal appear in.
Decals are a Generic Model category. You can create a sub-category to place decals on (it is an instance parameter).
But this is only useful to turn decals off in a view, Visibility graphics doesn't allow the sub-category to be set to transparent.
NOTE:
If decals don't appear at all in Realistic Visual style look at wiki page HELP for a solution.