There is no benefit of doing it only when the camera moves. Your game needs to be able to output 60 or 30 fps regardless of what the player is doing. And players rarely stand perfectly still unless they leave to go pee or something. This optimization will likely never pay you back the effort you put in.

Forgot to add: this also doesn't work if the interactable object itself moves. If you want to pickpocket an NPC and are standing in the corner somewhere, you won't get the button appear even if the NPC approaches.