Boom from top or bottom?
 

I know most people boom from the top down but occasionally I've seen people boom from the bottom.

Lets say two characters are standing up and having a convo on OTS form and the only way I can boom it is to mount my mic on on a tiny photography tripod on let's say a table beneth them. The other option is to use a light sttand with an arm and have the mic above them. It there an advantage in the sound from mounting it above them on a light stand then having it below them on table even if it's its the same distance from their mouths both ways?

I have boomed both ways. With mic proximity to subject the same, there was no sound diff that I could discern. I boomed from the bottom because of framing issues - Generally I boom from above. Cannot always do that though.

The advantage of booming from top is that you avoid reflections from the floor.

really, what floor you are talking about , the real one or one that on the ceiling :-)

the only benefit from the top ( if the frame aloud the both positions in the same distance )that the actors can work or talk with their hands .
and usially if they moove you almost cant do it .
outside where you have allot of urban noise the under booming will be better not to get the ground reflection

Hearing voices from above?
 

These scenarios are why God invented wireless mics.