Not sure how common this is-- but I love the look on it. I have been successful at adding some realism to my Beaufighter, Spitfire, and Hurricane by taking very small bits of masking tape, painting them flat red, and putting them over the gun and cannon ports on these planes. To make them look like they have been in battle, take a pin and poke a hole through them from the inside. A drop of glue to attach and it looks great!
If you can find yourself one of the very small Swiss Army knifes that they usually sell near the cash registers, you get a small set of scissors for trimming decals, a small tweezers, a toothpick for moving decals around, a knife, and a file.