My sensei emphasized to be a good uke you need to strike with intent and on target. So we had to learn to punch, elbow, knee, kick (basic strikes).
Also that when doing our wazas (techniques) that atemi (strikes) are always present.
There's a video/dvd by Shihan Mitsugi Saotome called "Oyo Henka." Where he goes over some strikes in Aikido.