Well Audi actually installed them both directions at the factory, which is fine for stock boost levels. Problem comes when the boost is increased putting too much pressure on top and under the diaphram. They last much longer when installed correctly with the boost going into the side port and the bottom port used as the return.
I used to run a Bosch 110 valve with no problem, put it in my friends car the same way his stock valve which turned out to be backwards and the 110 valve failed in 1 day even though he was running less boost then I was.