yorkshiregoth said:What's wrong with just turning the bike upside down on the floor whilst doing the necessary maintanence?
on possible reason is if you've got hope (or possibly others aswell, i don't know) brakes which have been non-factory bled then there may be an air bubble at the top of the reservoir. This isn't ever a problem normally as the level in the reservoir never gets even half low enough for any of the air in the bubble to get into the hose, but if you turn the bike upside down then you've got air in your brakes. Although my mate does it and doesn't seem to have a problem, it must just flukily enough all rise to the top of the reservoir again.
Other reasons you might scratch your handlebars, or damage anything that's attached to them.