You have to live by principles. One principle is honesty.
Was the accident reported to the police? Was the cost absorbed by the rental car insurance company? Did you have to pay anything? Is it on your record?
If the answer is YES to any of these question - you need to disclose it to your agent.
If you were not at fault and the rental company took care of it, and it is not on a police record, it was likely not reportable. So I would be inclined to not say anything about it. If you are ultimately challenged about it, you need to have the backup to show this was an incident, and you were absolved so you did not think it relevant - since it was not your car and the rental company took care of it.
If you have a good agent, you could always ask a what if question and see what he answers. Then explain the circumstances if he says you should.
Frankly, I don't think it would impact your rates.