I always remove the excess film. Cut the image right to the edge. Eliminates any problems with it showing later on, which I invariably have happen. If I can not cut out individual letters or numbers, as in a small serial number, I will cut the spaces between the letters at the top and bottom of the series so there is an uneven edge that makes the tendency to show not so noticeable. I will also paint the base color on the film between the letters / numbers to further hide the film, if at all possible. Take a look at photos in magazines. It may not be noticeable to the eye when looking at the model, but it seems to really bring out a decal that has the film on it when photographed.
My thoughts.