1) Your nav system will be more accurate if you hook up the VSS wire, I recommend you hook it up.
2) You need the parking brake wire hooked up to make changes to your nav system or view dvd's (either hooked up as per the owners manual or by doing the bypass modification) You don't need to hook up the reverse wire unless you are connecting a rear view camera.
3) I myself did the bypass modification, you're at risk of getting a ticket if you're playing dvd's while in motion, however the bypass allows passengers to enter nav data while in motion which is why I did the bypass.