Personally, I use Virtual Copy if I want to do something fifferent with a file that I already have processed, i.e., a black and white conversion, a different crop, or just to try something different on a file I already have PP'd to suit. I guess Snapshots would work just as well, I like being able to see the different versions in the Library which you can't do with snapshots. If something doesn't work out with the new version, I can just delete it. I usually stack the versions just to conserve screen space unless the new version is radically different or I just plain like it enough I feel it deserves its own thumbnail.
I don't send out prints so about the only use I have for jpegs is for the Web, so I have a folder named "Web Export" where I place all the LR exports. It's not in the LR catalog since I already have the original file cataloged. Periodically, I'll just go into it and delete everything. As easy as it is to export jpegs from LR, they're pretty much a disposable commodity.
If you're sending off jpeg's for printing, you might want to hold on to them until you get your prints back and are satisfied with them. Otherwise, I see no need to keep them around.