We have all heard at one time or another not to put egg shells down the garbage disposal.
Egg shells, in moderation, are actually good for the disposal. They are thin enough to travel around the blades and thick enough to push out mashed food and smelly bacteria that hide in the crevasses.
A word of caution: if you plan to make egg salad for a large gathering, you need to put the shells in your carry-out trash bin. A lot of egg shells can help plug drains.
In fact there was a recent warning from a building engineer posted recently:
Do not put egg shells through your garbage disposal! They lie in the bottom of your sewer pipes and will eventually clog. Use your carryout trash for egg shells or be ready to hand a plumber at least $150 to unclog your pipes.
Conclusion: egg shells may not be bad for your disposal but could help clog up drain pipes.