This is a good, no, terrific question that needs to be addressed. Emergency radios can be used for many purposes but their main design is to alert you of upcoming storms and potential natural disasters. In order for you to receive the alert for it, you must know you are being sent an alert. In other words, it needs to be placed in an area where you will hear the alert. This will vary from person to person but at night, your bedroom is a logical choice. If something occurs at night, the only way you will know is if the emergency radio wakes you from your stupor.
But, at the end of the day, it should be placed in an area that produces an optimal amount of reception. Wherever this happens to be, leave it there as a weak signal can render the radio essentially useless.