While there is not one answer to this question, here are a few things to consider when deciding where to place the Graphic Recorder:
Inform the speaker well in advance there will be a Graphic Recorder there to support their presentation.
If you place the GR at the back of the room, it can be distracting for the speaker to watch turning heads in the audience.
To maximize the value of the GR, allowing the audience to watch the art unfold is a critical part of the process.
A well lit area is important. Highly produced events may using a lot of moving lights, black lights (which cause color shifts), and pulsating lights. Be sure your AV crew is aware the GR will need clip on lights or portable lights for their work area.
Consider proximity to loudspeakers, and in general provide the GR with a location that has strong acoustics.
Placing the GR in a location that is easily visible to the majority of the audience allows everyone to access this additional modality of understanding and absorbing the information being conveyed by the speaker.
If the venue is light-filled, consider the movement of the sun throughout the day and where it might be problematic.
Consult with your GR before the event. A few camera phone shots of the venue can be very helpful to launch a discussion of the options for placement of the GR.