Keeping your own nape hair will help ease the transition (to touch), but in my opinion, the benefits of keeping hair in that area are negligible. Keep in mind that you will still have blending to worry about if you retain any of your own hair.

The one area that adds a great deal of realism that is very easy to blend is sideburns. If you keep some of your own hair in that area it's very easy to maintain and it goes a long way toward creating a realistic look.