Hi, George. This is NOT a valid assumption. The roof load has to be transfered down on the second floor just like it is in a one story house.
The wall in question may or may not be load bearing. There is no way to tell for sure without looking at the house or the plans or count top plates. One pretty good give away is that there is usually a wall on the first floor directly under a load bearing wall on the second floor.
Another way is to look for big beams that span the basement. If you have a wall on the first floor directly over a big metal or wood beam in the basement, you can be pretty sure that the wall is load bearing and then the above is usually true.
It's difficult to give hard and fast rules for every load bearing wall situation because creative framing can transfer loads in unusual methods so counting top plates is the safest bet.