The correct ear for a ID tattoo is the left ear. The right ear is reserved for the ARBA registration symbol or registration number, which can only be tattooed by an ARBA registrar. However, judges don't usually check the right ear from what I've seen, and they're not supposed to identify the rabbit until they're done placing and examining them - that's when they check the left ear for the ID. I personally probably wouldn't disqualify a rabbit for a tattoo in the right ear.
The ARBA standard says a disqualification is when there is "illegible tattoo, or tattoo not in the left ear."
If you get a rabbit with a tattoo in the right ear and not the left, and you intend to show it, you need to give it a tattoo in the left ear as well.