I'm kind of an audio snob so take or leave this advise.
Even with one ear, you can still hear stereo. Just like with one eye you still have depth perception. For that reason don't rip your music to mono if you plan on using the same MP3 files on a stereo at home, in the car...
Merging stereo channels can cause sound cancellation or introduce noise to the music. If you rip it that way you can't undo it.
For 1 speaker headphones, however, a merged L/R option would likely be the best plan. You may be able to make your own. You just have to twist the positive leads together and solder them back to one ear-bud.
The worst option is listening to one channel of stereo...... uuug! Many 60-70s bands put entire instruments in indivdual channels. Even modern recordings may place parts of the drum kit or most of an instrument in one channel. Hearing half music is maddening.