The most common question with decals is whether to clearcoat over them or allow them to stand proud of the finish. I am an advocate of clear-coating for several reasons. Decals are always clear-coated in the armor and aircraft modeling worlds, the reasons for which are scale film thickness and shininess. These same reasons hold true in the car modeling world.
Clear decal carrier should always be trimmed off as close to the artwork as possible. This is doubly true if you are choosing not to clear coat. No real-world cars have one inch of clear film surrounding the artwork. Visible clear carrier is another glaring "tell" that the subject is a scale model.