70 - 80 bpm for slower, more mellow feeling (especially skanking or bubbling)
80 - 90 bpm for cool running reggae/dancehall vybes
90 - 100bpm for up tempo bashement/ragga
Then again, it all depends on your mood and what you're trying to do. It's not carved in stone to use a set tempo. If the vybes is right use whatever you feel. Just remember to have fun!
Well, 140 for dubstep is really more like 70 bpm, right? (Cut it in half).
Dark Mage has some good advice there, but it's really whereever you're feeling it. Listen to the types of songs that you're trying to emulate, and find out their bpm. Then try to come up with your own groove at that tempo.