This depends on the use of the room. It is largely a subjective judgment by the occupiers on what level of noise they are comfortable with and will vary by location. For example, people generally only feel comfortable with a noise level of 30 – 40dB in a bedroom but will happily work in an open plan office with a noise level of 45 – 50dB. You would expect a background noise in a library to be around 30dB.
If you make a room too quiet, then you begin to listen for other noise sources that you would not have picked up before. Go for a noise level appropriate to the use of the room.