You can probably get away with not maintaining your neckline when you're just sporting stubble. But once it goes beyond 3mm and starts turning into a proper beard, it's time to start grooming. A well-defined and well-maintained beard neckline makes almost any style look better and tidier.
When should I shave my neckline?
The secret to maintaining a smart beard neckline is regular trimming. When the hair in this area has grown to 3mm or longer, it's time to shave it off.
Should I shave my neck when growing a beard?
If your beard doesn't cover your neck, shaving your neckline is still worth it. Doesn't mean you have to, by any means. You might prefer the shape of your beard without a shaved neckline.