Hardly anyone needs to cry on cue anymore. It's a skill that comes easily to some actors, but if it doesn't for you, you dont' need to worry. Lux and Baze had a crying scene and it was legit.

On stage, you have no need to worry, since your audience is not close enough to see you. What is most important is that you stay in the moment of the scene, and act from your heart during that scene. You'd be surprised; the tears may come naturally...