I do understand the storage VS sound quality argument, but here's what I reason - first of all, storage has gotten ridiculously cheap. I just bought a 750 gig drive for $225. So even at a bitrate of 320, you could store endless amounts of music for very little money.
But the main thing is, it's very very possible you may one day say "you know what, I've been listening to 128k MP3s for a few years now and I have finally decided that they DO suck and I can't stand it anymore!" Now you'll have to go back and re-compress your entire music collection at a higher bitrate. Also, I like to make DJ mixes, which means re-encoding a final mix; if I start with low bitrate files, the results will sound awful - if my originals are high bitrate, I can re-compress them and they'll still sound good.
Basically, using a higher bitrate to start with covers you for all forseeable circumstances; going with a lower bitrate to save a few megs of space will inevitably byte you in the end (ha ha, aren't I clever).