Simple question: Should OPTIONAL User Settings be Lazy-Initialized into the DB or always created with a new registration?

A user can set additional settings which are optional, so should a row for that optional setting be created for every user upon registration or only created when a user makes use of those settings for the first time?

Lazy-Initialization saves space, so I'm leaning towards doing it this way, but I'm not sure if there are any drawbacks.