Is it worthwhile to upload two versions of the same stock video at different frame rates?  Will there be any quality lost if a video frame rate is changed from 24 to 30 or from 30 to 24?

No. Stick with the standard. Most agencies recommend 30 if you are uploading NTSC.

I agree on keeping it native. Let the end user change the framerate if they need to. If you're talking HD, this day in age, you really don't need to worry too much about 24 (23.976) or 30 (29.97) if you're talking USA. While it is true that much of broadcast is still 29.97 fps, tons and tons of stuff are going 23.976. Film and digital cinema is predominantly 23.976/24fps.

I shoot and upload all of my stuff at 23.976. I don't worry about broadcast. Truth be told, most producers/buyers are looking at the content FIRST. That's the most important. Technical comes last usually.

And if you want to get technical, you're actually safer with 23.976. It's MUCH MUCH easier (and looks better) to convert 23.976/24fps to 29.97/30fps than the other way around. Try it some time and you'll see why. When you convert 23.976 to 29.97 you're essentially just adding a few extra frames (repeating frames) to get to 29.97. So you don't lose anything and most NLEs (I know Premiere Pro does) will handle this automatically and easily. Now if you downconvert 29.97 footage to 23.976, you're REMOVING FRAMES and thus your footage will look choppy and not as good as footage shot natively at 23.976. It's a much more complicated process to downconvert to 23.976, usually resulting in third party plugins, and even then it never looks quite as good as native 23.976 footage.

Make sense?