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.
> 
> 
>
There is no need for a explicit not-connected symbol. Don't connect anything to it, and nothing will be connected to it, and that will be obvious in the schematic. There really is no need to do anything more.

No, the DRC (Design Rule Check) doesn't care at all whether pins are connected. The ERC (Electrial Rules Check) only complains about floating input pins, but then again those should be connected to something.