I generally match the power outlet height.
Exactamundo. Other reasons aside, I like to make retro-work look like it's always been there. Match height, color, finish, etc., with existing devices.
Example: when adding a switch in a room for, say, a ceiling fan, you should change the existing box, adding a gang, rather than sticking a separate box.
Of course, if you're adding more than one switch, you might use a separate box, but you'd still match existing devices and plates, and height as well.