Well that's what I do.
There is some contention about whether the flat face faces toward primary drive or towards the middle of the crankcases. I've seen it written here that the flat face goes toward the centre of the crankcases.
The workshop Manual
is obtuse and uninformative on this saying "with the knife edge always towards that part which is to be sealed"
I have always put flat face to primary, but I keep losing primary oil. As I run a crank-vent which probably generates a vacuum in the crankcases I'll be trying the flat face in next time around.
I always put a bit of loctite sealant on it as the seal is a bit loose in the crankcase, but this is probably futile I suspect.
People with more practical experience of this will chime in soon.