Putting coffee grounds down your sinks garbage disposable is okay as long as you only put a little bit of coffee grounds down the disposable at a time.
Just be sure to leave your water running while you use the garbage disposable to force the coffee grounds down the sink drain.
You can also just dump some coffee ground directly down your sink drain without a garbage disposal as I do it all the time after making coffee.
When I dump the coffee grounds in the kitchen trash can the gnats and flies tend to hang around the trash can so by dumping the coffee grounds down my sink drain and washing them away with water it keeps the gnats and flies away.
You could also just dump the used coffee grounds out onto the ground or put them around some plants and flowers to help them grow.