I use all 3 techniques. Each is used for specific types of sandwiches. Both outsides for toasties, single for oil rich spreads eg peanut paste, meat spread, also for fish, ham, chicken sandwiches, salad and no butter for my fav which is oven fresh bread on it’s own, or for pork, roast beef or roast lamb.
Virtually never use butter or margarine on breads or toasts. The only exception is once in a blue moon raisin toast where butter (by itself) makes a better and perfect combination.
We do use condiments such as mustard, mayo and relishes/pickles regularly on savoury sandwiches (one side only)…which in some respects is a butter replacement.
Sandwiches in our flat bed toasted sandwich maker are also naked…they brown and crisp up perfectly well on the non-stick surface.