Is it okay to sing along with the performers at a musical that you love?
Yes, as long as you do not sing too loud.
No. It's not okay. If you absolutely can't hold it in, maybe just mouth the words to yourself. When I go to a performance, I want to hear what's going on on stage, not the audience. It's like talking at the movies; it's not fair to the rest of the audience, and it certainly isn't fair to the performers. Sing your heart out at home, by all means, but please keep it silent when you go out.