I'm absolutely positive I'm talking about bitrate and not sample rate.
With variable bitrate audio formats, what ends up happening if the audio fits completely within bitrates lower than you specified, the bitrate will actually be lowered by the encoder to save disk space at the cost of absolutely nothing.
The same thing happens in variable bitrate video. If you're giving the encoder more bitrate than it needs to store all the information and are allowing the encoder to run less if it needs less, it will use less when it can.
http://xiph.org/~xiphmont/demo/neil-young.html The "audiophiles" that choose the 192kHz sample rate are actually not as smart as they appear.