Well if it's a Mac Book Air tell them to take it back! Others with Mac Book Airs have reported 10.6.5 fixes some issues for them.
So if you are scared just clone your drive to an external and then do the update while not doing anything else while it is updating. If the update fails then just boot from the external and clone back to to the Air, easy enough.
One should NEVER compute without having a viable backup! Anything that cause that computer to crash and you loose data, it will be your fault for not having your data backed up.
Of course it is. As an actual bench technician, my experience has been that 100% of every "this update broke my machine" that I've personally seen has been bad hardware. The two big things that corrupt any install are bad RAM and a bad hard drive.
Granted, there *are* still bugs out there, but they don't kill machines. For example, with the 10.0.0 release of iTunes, there was a bug that caused the iTunes library not to be visible. It was still there--you just couldn't see it in iTunes. It didn't kill the machine. The first version of iPhoto '11 had a bug in it that might not upgrade your library correctly, but it doesn't kill your machine. If it's a bug, restoring from backup or reinstalling will fix the problem. Bugs don't kill machines.