I've read some condesending online critisisms of turning the bike upside down to work on it, but that is what I do, especially roadside. Sure one can learn to do some stuff keeping it upright or on its side and some stuff is better to do upright. edit: it was in the r.b. FAQ:
http://draco.acs.uci.edu/rbfaq/FAQ/8a.5.html
In my garage I have a carpet I turn bike upside down on.
I'm not so stupid as to turn it over on a computer or other damagable part or on nasty surface.
Al