Generally speaking spacers are not a good idea
But why you ask
Well first of all they take away that much lug nut engagement
There are other reasons, such as applying increased leverage forces on the wheel bearings. You can easily see this when guys (like me) change wheels which move the center of the wheel outward. This may or may not be a problem. You see it in the front end of Chevys with Unit bearings that don't last very long.
As for the lug nut engagement, most 3/4 ton and up trucks have very generous length lug nuts and usually of a 9/16" diameter. Taking away 1/2" of lug nut engagement wouldn't affect clamping force at all, so in your case, you're probably fine.