What is the difference between making contact with the spoon at the start of the process of shovelling food into your mouth or at the end? Unless the OP is suggesting that he removes the food in such a way that it isn't entirely cleaned from the utensil, leaving a protective layer as a 'barrier' so that no contact is ever made...
And if so, what about touching other parts of your mouth in the process? Surely an inverted spoon is more likely to touch the roof of your mouth. You have taste receptors all over the place, not just on your tongue. And what about the lips? If you really wanted to avoid the metallic taste of a spoon, you'd need to invert, paint the dessert onto the tongue while leaving a layer, and remove the spoon with an open mouth, not touching your lips.