Depending on how your cables are routed, turning the bike upside down can pinch cables. At best you'll scuff your saddle and hoods.
Unless you have a fixed-gear, you can clean and lube the drivetrain just as easily pedaling backwards with the rear wheel on the ground, as you can pedaling forwards. Where a workstand comes in handy is adjusting the derailleurs. But even that can be done by lifting the rear wheel off the ground by the seat tube - it's kind of a PITA and easier if you have a friend to help, but not that big a deal. If I just need a minor adjustment, it's actually less hassle for me to do it that way than to drag the workstand out.
It is a good idea to look at the underside of your bike periodically. Even though I'm pretty good about a periodic thorough cleaning, I had no idea how worn my bottom bracket cable guide was, until I disassembled my bike for shipping this fall. 
+1 on knowing how derailleurs and brakes work. That, plus changing a tube, will get you through most situations. Patching a tube is a good skill to have, too, because you never know when you might have more flats than spare tubes.
Last edited by OakLeaf; 01-11-2010 at 05:32 AM.
Speed comes from what you put behind you. - Judi Ketteler