What's the benefit of not soldering NC pins compared to soldering them to a disconnected pad?
When you solder to a pad -

Pro: -It is mechanically stronger

Con: -You will likely have a very, very minor increase in noise inside of the device, negligible in all but the most extreme situations (which you wouldn't be using a plastic device with NC pins anyway)

-You increase build time slightly and you also increase the risk of a solder short (or other process errors)

Anyway, I usually put a non connected pad there just in case I want to change ICs and wire a jumper wire over or something like that.