Is there ANY reason to push the stunt button other than to do pointless wheelies?

Thank you sir. :D

which button is use to do stunts

I remapped my controls system so you'll have to look under options because I don't remember the original.

Better late than never BUT...the original key is the space key. As IronArcturus said, it works as a boost key also - be aware that it's entirely possible to die from hitting it at the wrong time. If you hit the stunt key just as you you rear-end another bike, you'll most likely crash. What I like to do is just as I'm about to hit the top of an incline, hit the stunt key and you can, sometimes, get a GIGANTIC jump. I've actually passed more than seven cars while in the air AND you can also steer around corners and whatnot if you do it right. Takes a lot of practice but it looks awesome when you watch it in replay mode after winning.