Well, from my perspective, nips anywhere are always welcome. I love a good nibble or nip or bite. Around the neck, however... caution has to be taken. Even from a human. Don't ask me to explain the metaphysics of it, smarter minds than me have tried. But bites to the neck made with... intent cause a mark. Between lovers, it's a mating mark. Think of it as a promise ring that's a bit more visible and binding.

[He grinned down at her, leaning in and licking over her lip again.]

The specifics of them vary, but have you ever seen a mated pair of shifters with little marks on their neck. Ones that maybe might seem odd on a species that heals as quickly as we do.