Photography during a ballet performance is not allowed, only during general rehearsals. If you get an assignment to shoot a ballet, check in advance to find out if you are allowed to take photos, and if not what the alternatives are. Most of the time, you will be able to arrange something. In the end, everybody loves free publicity and wants to be in the papers.
Photographers are not allowed in classical concert theaters. The music is simply too delicate, and the clicking sound would be a major disturbance. When you see photos of an orchestra or conductor, they are usually made at the end of the concert when the audience is applauding. The doorman will let you enter the venue at the moment when the last notes are played. Of course, another option is to take photos during rehearsals. If you get the opportunity to shoot for an hour or so in rehearsals, you should always go for it. With a rangefinder camera and a very long lens, you may be able to convince the manager that you won't disturb anyone. However, the chances of getting permission are very small.
Figure 37.3. Janine Jansen, 2004. Fujifilm Finepix S2, 1/30 sec with fill flash, f/11, ISO 200, 135 mm
With modern dance performances or musicals, it is hard to do your job as a photographer. It all depends on how loud the music is played. The first thing to check is whether you hear the shutter while the music is playing. If you do, photography is hardly ever allowed. Before you ask for permission, always think of a couple of strong arguments to convince the person responsible.