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: 
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.
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.