One of the main problems effecting paintwork with fitting graphics / decals /stripes either by yourself or by the not-so-professional fitters (
just to make it perfectly clear "Phil aka The Pirate" is not one of these 
), is when they come to trim or cut the vinyl / graphic / decals on the car (with a scalpel) and cut into the actual paint
which can cause corrosion issues in the future and also marks should you wish to change or remove the graphics in the future, now that could cause warranty issues if you didn't get the Main Dealer to fit the graphics originally
.