Connecting any signal route with unused pin
Some manufacuters document their pins as "Do Not Connect" and "No Internal Connection" rather than just "No Connect".

You should not connect anything to a "Do Not Connect" pin. It is likely to adversely affect the behavior and performance of your circuit. For example, it might put the chip into a test mode, or change the behavior of an analog circuit.

In principle, you can connect whatever you want to a "No Internal Connection" pin without adversely affecting anything. But you might want to consider whether you ever want to replace the chip you're using with an alternate part (a faster one or a cheaper one from the same family), or if it's possible the chip vendor will change the design in a future revision so that that pin is no longer "NIC". 

If your chip's datasheet just indicates "No Connect", you might be able to get more information by asking the vendor's apps engineers, but it's probably safer to just not connect anything to that pad.