I always let mine warm up at least a couple minutes, or more depending on the temperature, especially first thing in the morning. As long as you do this there will be no harm to your engine.

Just don't be like those idiots back when I was in tech school and rev your engine all the way as soon as you start it and then take off like a bat out of hell...I'm sure that caught up to them eventually, but hey it was probably coming out of mom and dad's pocket so why would they care, right!