I usually make my brushes in Photoshop CS and Photoshop CS5 and they works fine in CS, CS4 and CS5 and in Ps Elements 5.0 and higher. I don't know about Photoshop CS2, CS3, but I know that other people are using them without complains. The problem might be that big brushes 2500px won't work in the previous PS versions because of size restriction.
As far as Photoshop 7 is concerned, *AscendedArts wrote very useful software ABRMate [link] that allows you to convert PS CS brushes into PS 7 in one click.
That leaves us only with PS 6.0 brushes.
I'm still thinking that maybe it's better to divide brushes by type, the same way as stock photos: Nature, buildings, people, objects, weather and so on.
Versions are nice, but you don't need all of them. All you'd really need would be PS 7-CS2 and then PS CS3 + and possiably Elements. It was only after CS3 that brushes started to have those silly compatability issues with past versions.