i may get flamed for this, but i think only the best quality is acceptable if you're selling the files. i use lame to encode at 320 constant bitrate, and that's the only way i'll play the files in a live set. if it's just for listening, then lame with the --alt -preset standard switch should be OK. i think it's important to avoid crap sounding MP3s, as that's how the format got a bad name to begin with.