The challenge is not to overexpose the highlights; these are usually on the face and it is really hard to get the skin tones looking natural if the exposure is too far out.
Easy rule is to expose for the face and the rest of the image can look after itself.
If the performers are spotlit, then the exposure can be remarkably consistent and I'll opt for Manual exposure.
Auto exposure can be very hit and miss under high contrast stage lighting; I do use it but always check the RGB histogram carefully to make sure the exposure is OK - you can't rely on looking at the screen.
My settings for shooting theatre / concerts are usually:
- Aperture wide open
- Shutter speed at least 1/125 otherwise the performers will have too much blur.
- ISO whatever is needed to achieve the above! Typically it seems to end up in the range 800 to 2500