1. Consider the energy and environment of the scene. Sitting down is a relaxing and grounding physicality. So when playing scenes that require a stronger energy such as in half hour comedies and workplace scenes where your character is in a state of physical momentum, standing up may be the better choice. For example, if the scene takes place in the workplace as in many procedural dramas and your character is physically on the move (such as walking down the hospital hallway having a conversation about a patient with strong intent and pace) then you're better off playing the scene standing up in order to communicate that energy. If, on the other hand, you're playing the same character who's having a relaxed conversation with a friend over dinner after work, then delivering the scene from the chair is likely the better choice.
2. Consider the placement of the camera. If you walk into the room and see that the camera is placed on a tripod in a low fixed position facing the chair (in other words there is no camera operator behind it to raise it up) then it's best to sit during the scene even if you had planned to stand. If, on the other hand, there's a camera operator present then go ahead and stand knowing that they will adjust the camera. Also, when a camera operator is present you do not need to inform them of your movements during the scene, just trust that they will follow you, that's what they're there to do.
3. Simply because the chair has been placed there for you, doesn't mean you have to use it (usually). Most of the time when you walk into the room, a chair will be placed in the space available for you to use. This is a professional courtesy on the part of the casting director in case the actor prefers to sit for the scene. If, however, you've decided to stand then the best thing to do is simply move the chair out of your way before the scene starts. You don't need to ask permission to move the chair, just go ahead and move it gingerly and respectfully out of your space. That's part of what it means to own your space in the room. Now, occasionally you may happen upon that casting director who placed the chair for you and insists that you sit for the scene. This is not the norm, this is the exception. And if this does happen, it's better in that situation not to get into a debate with the CD but to happily go with the flow and adapt to the chair for the scene. Remember, the CD is the gatekeeper to getting seen by the producers and they may want you to sit for a very good reason, so just go with it.
4. Avoid up and down movements from standing to sitting and vice-versa. Smart auditioning, especially for TV, is economical auditioning. In other words, the audition is not a stage it is a space. So if the scene calls for your character to go from sitting to standing (for example, in the scene the writer's direction states: "Sarah can't believe what she's just heard. She stands up, indignant.") do not become obligated to this direction. What's important is that Sarah is emotionally charged by what she's just heard, it's not important that she stands up as a result of that. Any physical movement in the scene should be driven by specific purpose and intent, otherwise it can occur as random. I advise going from sitting-to-standing or standing-to-sitting during the scene only if it is absolutely necessary and critical to communicating the environment of the scene. For example, in the beginning of the scene Charlie arrives for a job interview, then the scene segues into the job interview itself where Charlie is sitting across from his potential employer. In this instance, starting the scene standing and then sitting for the job interview can make sense and doesn't feel overly "stagey".