Personally, I love the challenge of taking photos in a low-lit theatre with no flash photography. This is where a 50mm prime lens or something else with a wider apeture (f/1.4-1.8) can assist with that by bringing in more light into the lens. The stage lights will play with you and be annoying for some moments, but when timed right, they will work in your favor. Setting the camera at a higher ISO for your shutter speed helps as well. I also used a 75-300mm zoom lens (f/4-5.6) for some shots, but you can see the difference in quality a wider apeture can give.
Click on thumbnails to view full-size images