There is so many different ways and subjective taste palettes that I don't think there is one way. When I was younger and in a rush, I always drank my espresso maybe 30-60 seconds after they made it, the heat became part of what I enjoyed (eventually). Now I let it cool down to just above room-temperature if I know the place has good beans/roast as I feel even more flavor then. If I'm drinking a Americano, then it pretty much ends up room temperature before I even start drinking it.

Then we have my neighbor who I end up at the same cafe in sometimes, who poor their espresso into a glass of ice, then drink it, and they seem to enjoy their coffee just as much as me, so in the end, I don't think there is a right way here :)