There's a nubmer of reasons for goign upside down.
There's the fact taht you'll usually lift your head, clearing the airways for singing.
Also, it makes it easier to see any music/cue sheets that the vocalist might need.
A good tube mic shouldn't need to be upside down- it shoudl ahve been designed tso that the heat from the tube either doens't effct the capsule, or does so in a pleasing way. However, I mount cheaprer tubes upside down, because you just never know.
Also, ribbons are occasionally mounted upside down, in an attempt to "balance" out the sagging that occurs whilst they are right-way up.