In order to get the optimal exposure some form of exposure metering can be very useful, but often has to be interpreted or compensated.
The problem with all exposure meters is that they make the fundamental assumption that the scene that they are measuring is equivalent to a uniform grey with a reflectance of 18%, i.e. an average scene. In many instances this works OK. However, with theatre photography we often encounter situations when this is not appropriate. For example, if we have a well lit actor in front of a dark background then the meter will attempt to render the black area as grey, thus over exposing the actor in the foreground. The converse also applies. if the background is of light tone and very well lit then the meter will try and render it as grey, and so will under expose the actor in front of it.
With this knowledge then we can compensate for this, either when we use all manual metering or by dialling in exposure compensation for aperture or shutter priority automatic metering. The amount of compensation required will vary depending upon the situation but is typically between 0.5 to 2 stops. Those with digital cameras are at an advantage in that they check the the appearance of the image on the LCD panel on the back of the camera and also examine the histogram to see if the degree of compensation is appropriate.
Another factor to consider is how much of the frame the meter uses to take it's reading. A built in meter in a modern camera will typically have a variable field of view. From smallest to largest these are spot, centre-weighed and matrix. Spot metering looks at only a tiny spot, ignoring the rest of the frame; Centre-weighted uses a larger circular area to make most of its reading whilst matrix metering uses the whole frame, but in the most sophisticated systems, makes inferences based upon the pattern of light an dark areas it senses. The degree of compensation required for each of these will vary, and will ultimately depend upon your experience with the camera, metering modes and situations. So the best thing to do is to use the camera and meter under very varied lighting set ups, study the results, and compare them to the exposure either recorded in the EXIF data if shooting digitally or that you have noted down, if using a film camera.