Are you sure they are "bent" or you are just seeing the shaping and profiling that is done intentionally to improve shifting performance? If all five of your bikes show this, either you are mis-interpreting what you are seeing or your all are incredibly harsh on your equipment.
I've ridden 100,000+ miles on Shimano cassettes and freewheels and am familiar with dozens of other high-mileage Shimano cassette equipped bikes and none of us has ever bent or broken any cog teeth.