The ultimate answer for this is: It depends on what material you are encoding.
And the strongest scientific evidence is in the coders themselves. When encoding an mp3 using VBR (Variable Bit Rate), some encoders show how many frames where encoded using which bit rate. Here's a screenshot from LAME:
You will notice that only 10 frames out of 10735 were encoded using 320kbps, which is the highest bitrate mp3 supports. Had the quality been set to highest (which is not in this example) this would practically mean no audible differences should be heard. Had no frames been encoded using 320kbps, no audible differences should be discernible. Sadly, the output doesn't mention whether the 320kbps encoding was sufficient, or whether a higher bit rate would be needed.
In my experience, only dense and harmonically-rich masters put load on the 320kbps. Most vocal tracks, or bass recordings would rarely require more than 192kbps.
From various experiments that have been conducted, including some I have been part of, you should expect to find that our hearing is not aligned with the output of these coders. In other words, with everyday type of material and environments, people cannot tell the difference between 192kbps and 256kbps, let alone 320kbps; while many experiments are not comprehensive (testing environments and statistical analysis), there is strong indication that our perception is much worst than assumed by mp3 encoders. Bear in mind that most digital radio stations use 96kbps for transmission.
However, a final verdict should be made on a case-by-case basis using rigorous research, which is extremely hard to conduct. Alternatively, the process above should give you cheap and fast indication to whether it should even be possible for people to tell the difference.