I'm not positive about this, but why would it make a difference?
The heater core wouldn't really be directional if you think about it. It's basically a length of copper tubing that's been bent & stuffed through some aluminum fins.
On mine, the (3/4") hose coming from the water pump is connected to the upper/outboard core barb. The lower/inboard barb is connected to the (5/8") hose leading to the intake manifold.