Question: I looked around for NC symbols etc. and off course, leaving pins hanging will throw errors when doing a design rules check. So what would be the best practice for indicating NC pins in Eagle CAD?

Answer: 
I asked the same question of Cadsoft support only last week. I mentioned that I prefer to have a symbol on the schematic so that I know that I've properly reviewed that the pin in question can be left unconnected, the response I received was:

> 
> We don't have an equivalent 'X' symbol in EAGLE. You could create your
>  own no-DRC symbol without a footprint, if you define it as an external
>  device it won't raise an error for not having a footprint. So that's a
>  feasible solution.
> 
> 
> 

After finding that I couldn't add a schematic symbol without a package their follow-up was:

> 
> See the HELP pages for the ATTRIBUTE and PACKAGE. The key to defining
>  a device as an external device is to create an attribute called
>  *EXTERNAL* in the device. This will allow you to create a part that doesn't need a package. This is handled in the library.
> 
> 
>
NC symbol can be very useful when Net classes are used. It is way how to create net with different net class (e.g. clearance matrix) for not-connected pin. General NC pin has default net class.