Anyone who has ever worked as a background performer (generally referred to as "extras") on a film set or television show in Los Angeles has probably heard other extras talking about the stigma attached to people who do extra work on any kind of regular basis. The logic behind this stigma is based on the idea of directors associating extras as simply non-actors who fill the background of a given scene. There is a very common belief in Hollywood that if someone is recognized by a director as a person that was seen working as an extra, the director will only view that person as an extra, and will not take them seriously and hire them as a principal actor.
Talent Agents that represent actors in Los Angeles discourage actors from doing extra work, and often advise getting a flexible job such as waiter or sales person instead. There are four reasons that agents give talent this advice. One reason is because talent agents do not receive any commission from extra work. Second, extra work is not truly flexible. Once an extra is booked for work, they must commit to a full day - almost always including overtime exceeding 12 hour days, including nights and back-to-back days. This does not allow the availability to the agent for auditions, interviews, call-backs, etc. Third, the talent agent does not want their clients being pigeonholed as a professional extra by directors. And finally, the talent agent (and casting directors for that matter) would rather see actors taking workshops, scene study classes, and doing live theater.
With all that said, there is still a valuable reason that aspiring actors do at least some extra work. Being on a film or television set can be a terrific way to become comfortable in that type of environment, as well as learn the ins-and-outs of how a production comes together on a real professional working set. Knowing what is expected of actors either in a sound stage or out on-location is of the utmost importance. A good deal of knowledge and comfort can be gained by working as an extra in different settings such as on a sitcom before a live audience, a soap opera, feature film night shoot on-location, or a television commercial that typically shoots 50-75 takes per angle.
This extra work can be done discreetly without being singled out by a director, or having your talent agent knowing about it. It should be done sparingly, preferably even before signing with an agent. Many extras try to get "camera time" by intentionally placing themselves in a visible part of the camera's frame. This practice can get you noticed by the director or your agent. It is best to lay low and simply observe what is going on and becoming confident in the environment, getting rid of any stage fright, etc.