I think there are better way to keep pieces organized.

I don't know about that. I mean, Opus numbers tell you little about the pieces themselves, and it's just an indication of work order (but not always).

I really like and agree with what Gardener said, although I do go a bit further and also write down the place where I finished this composition as well as the time. I think this gives an extra something on the date-stamp.

I actually first saw this in someone else's composition (for voice and electronics) where he had noted down the time, date and place, and it instantly kid of made me feel in the place of the composer when finishing the piece (it was morning, in Bangor, Ireland). So I think it will be interesting to see how I'll feel when in a few years' time I'll be going through my uni stuff and see this score I finished yesterday, and remember how I had stayed up all night drinking tea and eating leftover pizza (home-made, please :P) finishing the last piece for the year..

If in a few years' time I find out that I'm wrong, then I'll simply stop doing so and stick to the date :P But too much information is always better than not enough information :X

Finale date stamps your files and you can add additional information to the file if you wish - that works for me. :)