Rounded corners make it far more difficult for little fingers (think persistent children inside of school buses, or bored adults on a train or airplane) to pick them off. It also makes it harder for the wind to catch the corners and slowly peel them up. Rounded corners make your decals look better and last longer.
At Muir Omni Graphics, this is the only time we cut corners in our process.
When designing decals, consider not only their application but how they interact with people and the environment.