I'd say wait until the next morning and take your dogs out to track it. Don't try "finishing it off" with a knife. I know in the state of PA you are not supposed to shoot after dusk and before dawn. But I'm sure they wouldn't fault you for the "ending shot" after dusk if you tracked it and found it laying there still alive. After all it's a law (atleast in PA) that you must make an effort to recover your game. Can't recover it if it's still alive and wounded.
Cutting the neck on a trophy just ruins the mount and screams amature. You might get away with a small head mount but I look at my first one every day and am reminnded of the stupidity of "cut his throat".
I'd have to see if the taxidermist would make up a special mount and intergrate the knife into the throat of the mount. That would make quite the conversation piece.