It depends.
If one of your streams tends to have or give off air or gas, you want to enter from the bottom and leave from the top so that the air/gas can be swept along and out of the Hx.
Then your other stream needs to go conter current or as counter current as possible to that stream.
Sometimes you are just stuck and have to make the best of the compromises you have to make because of equipment arrangement, etc.
If I have a water stream for example and I know that on heating up it wants to liberate air, I will try to have it flow from low to high.
rmw