As the first person said, it all depends on your attention to detail.
The average listener doesn't care and the average listener's speakers also cannot playback the differences in quality.
Lower bitrates cancel out silence and extremely subtle tones, thus creating smaller files. 190 kb is a desirable medium for mp3 (capacity savings and quality). I always prefer 320K, because the audio quality is almost like CD quality.
Songs that will sound worse on lower bitrates are songs that are heavily compressed or not compressed properly. What I mean by compression has nothing to do with the file itself, but rather, the technique used by audio professionals to obtain the loudest possible final recording / project. Songs that are slightly distorted will have a more audible pumping effect at lower bitrates, wheras higher bitrates of the same file source will sound better because all the extra audio that does not get cancelled out by the lower bitrate will fill the sound better that was heavy compressed in production.
Since you have BOSE speakers, I recommend going for the hihger bitrates even more, because you will notice the lower quality and graininess that the lower bitrate files will output. If you've ever listened to a good vinyl record or well produced CD-quality music, going under 320K will be detrimental to your sound. MP3 is already a reduction in quality, so stay at 320K.
If storage becomes a concern, go down to 192, or 256.